Line sharing multipoint pots splitter amplifier-based coupler

ABSTRACT

The eavesdropping prevention system and method of the present invention provides for shunting leakage signals in a communication system. The amplifier-based coupler is constructed with amplifiers configured with a nearly-zero impedance path which effectively prevents the propagation of a leakage signal onto other communication connections which are coupled to the same communication device. Leakage signals are attenuated due to the nearly-zero impedance path, rather than propagating through the higher impedance communication connections.

CLAIM OF PRIORITY

This document claims priority to and the benefit of the filing date ofco-pending and commonly assigned provisional application entitled “LineSharing Multipoint POTS Splitter” assigned Ser. No. 60/182,807, filedFeb. 16, 2000, and hereby incorporated herein by reference.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending and commonly assigned U.S.patent applications entitled “Line Sharing Multipoint POTS Splitter withIntelligent Termination” Ser. No. 09/749,338, filed Dec. 27, 2000, “LineSharing Multipoint POTS Splitter Masking Noise” now issued as U.S. Pat.No. 6,775,355, issue date Aug. 10, 2004, and “Line Sharing MultipointPOTS Splitter Controllable Line Selector” now issued as U.S. Pat. No.6,771,740, issue date Aug. 3, 2004, which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates generally to data communications, and moreparticularly, to a system and method for enabling a multiple linecommunication device to communicate over a plurality of differentsubscriber loops in a manner which prevents a potential third-partyeavesdropper from detecting a leakage signal.

BACKGROUND OF THE INVENTION

With the increasing bandwidth demands from the advent of the Internet,service providers have looked for ways to increase data transmissionperformance over the copper wire local loop transmission lines thatconnect telephone central offices (COs) to customer premises (CPs). Inconventional telephony networks, customer premises equipment (CPE) arecoupled to CO switches over the above mentioned transmission lines,which are commonly known as “local loops,” “subscriber lines,”“subscriber loops,” “loops,” or the “last mile” of the telephonenetwork. In the art, the term “line” and “loop” are usedinterchangeably, both terms referring to the copper wire pair used in atypical telephone transmission line conductor. Historically, the publicswitched telephone network (PSTN) evolved with subscriber loops coupledto a telephone network with circuit-switched capabilities that weredesigned to carry analog voice communications. “Central office” or “CO”means any site where a subscriber loop couples to a telephony switchingunit, such as a public switched telephone network (PSTN), a privatebranch exchange (PBX) telephony system, or any other locationfunctionally coupling subscriber loops to a telephony network. Digitalservice provision to the CP is a more recent development. With it, thetelephone network has evolved from a system capable of only carryinganalog voice communications into a system that can simultaneously carryvoice and digital data.

Historically, the POTS subscriber loop was designed with the functionsneeded to communicate analog voice-conversation signals and subscriberloop signaling. The CO switch uses subscriber loop signaling to notifythe customer premises about events in the telephone network, whilecustomer premises equipment (CPE) use subscriber loop signaling toinform the CO to perform actions for the customer. Some examples ofsubscriber loop signaling include: the CO switch signaling to the CPEthat an incoming call has arrived by ringing the phone, the CPE (e.g., atelephone) signaling to the CO switch that the CPE is initiating a callby an on-hook to off-hook transition of the telephone handset, and theCPE signaling to the CO switch that a call should be connected to alocation by transmitting the phone number of the location.

Because of the prohibitive costs of replacing or supplementing existingsubscriber loops, technologies have been implemented that utilizeexisting subscriber loops to provide easy and low cost migration todigital technologies. Subscriber loops capable of carrying digitalsignals are known as digital subscriber lines (DSLs). Various digitaltechnologies provide customers with additional flexibility and enhancedservices by utilizing frequency-division multiplexing and/ortime-division multiplexing techniques to fully exploit the transmissioncapability of a subscriber loop. These newer DSL technologies providedigital service to the customer premises without significantlyinterfering with the existing plain old telephone service (POTS)equipment and wiring by utilizing portions of the available frequencyspectrum not used by a POTS signal. These portions of the frequencyspectrum are often referred to as “logical channels.” Logical channelswithin a subscriber line that carry digital signals are known as “DSLchannels,” while logical channels within a subscriber line which carryPOTS analog signals are known as “POTS channels.”

DSL technologies, such as but not limited to integrated services digitalnetwork (ISDN), high-bit-rate digital subscriber line (HDSL), HDSL2 andsymmetric digital subscriber line (SDSL), utilize different frequenciesof the available frequency spectrum and therefore do not coexist with aPOTS signal, which typically utilizes the 0–4 kilohertz (KHz) portion ofthe available frequency spectrum. These DSL technologies accomplish thisfunctionality by frequency-division multiplexing (FDM) a single datasignal onto a logical channel above (at higher frequencies than) the 0KHz to 4 KHz frequency range used by the analog POTS signals. Suchmultiplexing techniques and terminology are common to those skilled inthe art, and are not described in detail herein.

Several variations of new multiple channel DSL technology exist, suchas, but not limited to, Asymmetric Digital Subscriber Line (ADSL), RateAdaptive Digital Subscriber Line (RADSL), Very High Speed DSL (VDSL),Multiple Virtual Lines (MVL™) and Tripleplay™ with this group generallyreferred to as xDSL. Communications systems employing xDSL technologymay multiplex a plurality of data signals and a single POTS signal ontoa single subscriber line. An xDSL system employing frequency-divisionmultiplexing would multiplex a plurality of data signals onto acorresponding plurality of logical channels, each logical channelutilizing a different portion of the available frequency spectrum. AnxDSL system employing time-division multiplexing would multiplex aplurality of data signals onto a single logical channel with eachdifferent data signal allocated to a predefined portion of time in apredefined, repeating time period.

For example, an xDSL system employing time-division multiplexing of fourdata signals would subdivide a predefined time period into foursub-periods. Each one of the four data signals would be allocated to oneof the four sub-periods. During the first sub-period, the first datasignal would be communicated across the subscriber loop. During thesecond sub-period, the second data signal would be communicated.Likewise, the third and fourth data signals would be communicated duringthe third and fourth sub-periods, respectively. When the fourthsub-period has ended, the predefined time period repeats, and the firstdata signal is communicated during a new first sub-period. Thus, fourindividual data signals can be transmitted sequentially by allocatingone of the signals to one of the four sub-periods.

FIG. 1 is a simplified illustrative block diagram of a portion of anexisting telephony system 20 which includes a telephone company CO 22coupled to a CP 24 via a single subscriber loop 26. Subscriber loop 26may be any suitable connection for communicating electrical signals, butis typically a copper wire pair, as is well known in the art, that wasoriginally designed to carry a 0–4 KHz analog voice channel (POTSsignal). Located within the CO 22 is the CO telephony POTS switchingunit 28 which communicates POTS signals with the telephone(s) 30residing in CP 24 via the subscriber loop 26. In some instances,filter(s) 32 may be coupled between subscriber loop 26 and telephone(s)30.

CO digital equipment 34 and low pass filter 36 may be added at the CO tofacilitate transmission of digital data. Digital equipment 34 transmitsand receives data signals over subscriber loop 26. When a copper wirepair is used for data signal transmission, the wire pair is oftenreferred to as a digital subscriber loop (DSL).

Low pass filter 36 separates, or splits out, the POTS signal fordelivery to POTS switching unit 28. Low pass filter 36 is designed topass the 0–4 KHz analog POTS signal. In some applications, a POTSsplitter (not shown) may be used. Such a POTS splitter may also includea high pass frequency filter designed to pass the data signals, whichutilize the portion of the available frequency spectrum above 4 KHz, tothe digital equipment 34. Thus, a POTS splitter may split off the datasignal from the subscriber loop for delivery to digital device 38,thereby separating the data signal from the POTS analog signal. POTSsplitter technology is well known in the art, and is therefore notdescribed in detail herein.

Located within the CP 24 may be a plurality of digital equipment devices40 which transmit and receive data signals over subscriber loop 26.Illustrative examples of digital equipment devices 40 include, but arenot limited to, facsimile (FAX) machines, set top boxes, internetappliances, computers, personal computers (PCs) or the like. A digitaldevice 38, such as a modem or the like, is coupled to or can beinterfaced with the digital equipment devices 40 and subscriber loop 26.Digital device 38 may communicate with the plurality of digitalequipment devices 40 via an ethernet 42, other local access network(LAN), or the like. Alternatively, digital device 38 may communicatewith a single digital equipment device 40 via a cable (not shown). Forconvenience of illustration, digital device 38 is shown as being aseparate device. However, digital device 38 may be incorporated into adigital equipment device as a component.

Digital device 38 decodes a data signal received from the CO digitalequipment 34 and transmits the decoded data signal to the digitalequipment devices 40. The digital device 38 also encodes data signalsreceived from the digital equipment units 40 into a data signal fortransmission to the digital equipment 34. Modulation schemes used tocommunicate between CO 22 and CP 24 may include, but are not limited to,carrierless amplitude/phase modulation (CAP), quadrature amplitudemodulation (QAM), Discrete Multi Tone (DMT) or pulse amplitudemodulation (PAM), and are commonly known in the art and are notdescribed in detail herein.

FIG. 2 is a simplified illustrative block diagram of a portion of anexisting telephony system 20′, which includes a telephone companycentral office (CO) 22 having POTS switching equipment 28, low passfilter 36 and digital equipment 34, coupled to a customer premises (CP)24, via a subscriber loop 26, employing multiple channel DSL technology.

With multiple channel DSL, the plurality of digital devices 38 maycommunicate concurrently with digital equipment 34 employingtime-division multiplexing. For convenience, only four digital devices38 coupled to four PCs 40 are shown. Also, only two telephones 30 andfilters 32 are shown. However, any number of either digital devices 38or telephones 30 could be coupled to subscriber loop 26.

With the system of FIG. 2, any number of the PCs 40 may be concurrentlycommunicating (within their allotted time period and/or allocatedband-width) with digital equipment 34 at the CO 22 using time-divisionmultiplexing and/or frequency division multiplexing. Also, one or bothof the telephones 30 may be communicating with other telephones (notshown) through POTS switching unit 28 at the same time that the PCs 40are communicating with digital equipment 34 because the PCs 40 andtelephones utilize different logical channels, as described above. Ofparticular interest is that two people may each be simultaneously usingthe two telephones 30, such as in a conference call. Because the CP 24is typically under the ownership and/or control of a single customer,conference calling is acceptable from a convenience and securityviewpoint. That is, eavesdropping at the CP 24 is not generally ofconcern to the CP owner, and if so, the CP owner would be responsiblefor taking the appropriate safeguards and for implementing any securitymeasures to prevent undesirable eavesdropping at CP 24.

With the advent of multiple channel DSL technology, attempts have beenmade to couple a plurality of different subscriber loops to a singlemultiple channel DSL digital equipment unit, thus coupling a pluralityof different CPs to a single multiple channel DSL digital equipmentunit, such as multiple virtual line (MVL) technology or the like. Forconvenience, multiple virtual line technology will be referred to asMVL, such use of the phrase MVL is intended to encompass all forms ofmultiple line technology. FIG. 3 is a simplified illustrative diagram ofone such possible system 20″. MVL transceiver unit 60 is similar infunctionality to the digital equipment 34 (FIGS. 1 and 2) in that MVLtransceiver 60 encodes and decodes data signals which are transmitted toor received from digital devices 38A–38D. However, MVL transceiver 60may have other advantages and features (which are not described indetail herein because such features and advantages are not relevant tothe functioning of the present invention described hereinafter).

Four customer premises 24A–24D are coupled to CO 22 via four differentsubscriber loops 26A–26D, respectively. For convenience, a singletelephone 30A–30D resides in each of the CPs 24A–24B, respectively, andis coupled to POTS switching unit 28 to provide connectivity to theoutside communication system. PCs 40A–40D are coupled to digital devices38A–38D, respectively, and communicate over subscriber loops 26A–26D.Telephones 30A–30D also communicate over subscriber loops 26A–26D,respectively, through filters 32A–32D, respectively. Low pass filters36A–36D, or POTS splitters in some applications, provide for splittingoff the POTS signal to the POTS switching unit 28 and for splitting offthe data signals to the MVL transceiver 60.

The application of MVL technology, as illustrated in FIG. 3, has oneundesirable aspect that has at least one heretofore unaddressed need.This need arises from the fact that the POTS switching unit 28 at the CO22, via subscriber connections 26A–26D, concurrently provides service totelephones 30A–30D, respectively. These telephones 30A–30D areelectrically coupled to each other through a high impedance path viatheir respective subscriber loops 26A–26D, and the MVL transceiver 60.The high impedance path is such that when two or more persons aretalking on two or more telephones 30A–30D, respectively, the audibleinterference between the telephones is generally negligible. However, asillustrated in FIG. 4A and FIG. 4B, a small amount of a POTS signal,referred to hereinafter as a leakage signal, may be communicated fromone of the telephones onto the other subscriber loops.

For example, a person talking on telephone 30A may be sending/receivinga POTS analog signal over subscriber loop 26A (FIG. 3). Because low passfilter 36A may not be entirely efficient in splitting off the POTSanalog signal associated with telephone 30A, some of that POTS signalmay be detected on connection 62. This leakage signal may also propagatethrough low pass filters 36B–36D and may be detected on subscriber loops26B, 26C and/or 26D. Although the amplitude of the POTS analog signalfrom telephone 30A is not sufficiently great enough to interfere withanalog communications from telephones 30B–30D, this leakage signal fromtelephone 30A may be nonetheless detectable in some situations.

Moreover, in the above-described illustrative example, the user oftelephone 30A at CP 24A typically does not want his telephoneconversation detectable by a third party who may have access tosubscriber loops 26B–26D. That is, the user of telephone 30A typicallydoes not want their conversation being communicated over subscriber loop26A to be eavesdropped on. For example, the user of telephone 30A may bea stockbroker or security analyst who may be discussing confidentialinformation. An eavesdropper may desire to eavesdrop on the conversationto gain access to the potentially valuable confidential information.Such an eavesdropper, having access to one of the subscriber loops26B–26D, could detect the leakage signal with appropriate amplificationequipment such that the conversation on telephone 30A could beoverheard. Thus, there is an heretofore unaddressed need to prevent athird party eavesdropper from overhearing leakage signals that may existon subscriber loops which have been coupled into a common multiplevirtual line (MVL) transceiver 60.

FIGS. 4A and 4B are simplified illustrative examples of theabove-described situation wherein a leakage signal (FIG. 4B) associatedwith a telephone conversation (FIG. 4A) being communicated acrosssubscriber loop 26A (FIG. 3) may be detectable on subscriber loop 26D.FIG. 4A illustrates the available communication system frequencyspectrum 70 for subscriber loop 26A. The POTS channel utilizes a portionof the available frequency spectrum from approximately 0–4 KHz. Theconversation of the user of telephone 30A would generate an analog POTSsignal 72 as shown in FIG. 4A. (For purposes of convenientlyillustrating the various signals shown in FIGS. 4A and 4B, the signalamplitude axis has not been numbered. One skilled in the art willrealize that any appropriate axis numbering system could have beenemployed, and that such a numbering system is not necessary to explainthe nature of the leakage signal.) Also shown in FIG. 4A is a datasignal 74. Data signal 74 would be a data signal transmitted/received byPC 40A (FIG. 3) over subscriber loop 26A, through digital device 38A andMVL transceiver 60. This data signal occupies a logical channelutilizing a portion of the available communication frequency spectrumbetween a frequency of F1 and a frequency of F2. (One skilled in the artwill appreciate that the actual frequency values F1 and F2 need not bedescribed to explain the nature of the leakage signal.)

FIG. 4B illustrates signals on the available communication systemfrequency spectrum 76 on subscriber loop 26D (FIG. 3). Data signal 78 isthe signal transmitted/received by PC 40D over subscriber loop 26D. Datasignal 78 occupies a portion of the available frequency spectrum from afrequency of F3 to F4. (One skilled in the art will appreciate that thefrequencies F3 and F4 need not be specified for an understanding of theleakage signal, and that frequencies F3 and F4 may or may not correspondto frequencies F1 and F2 of FIG. 4A depending upon the characteristicsof the MVL transceiver 60 and the particular multiplexing schemeemployed.) Leakage signal 80 is shown to be present on subscriber loop26D on the POTS analog channel (0–4 KHz). Leakage signal 80 isassociated with the analog POTS signal 72 of FIG. 4A. Leakage signal 80is seen to be a low amplitude signal, being only a fraction of theamplitude of signal 72 (FIG. 4A) and thus, is seen to be of asufficiently low amplitude such that leakage signal 80 would notsignificantly interfere with telephone conversations on subscriber loop26D (FIG. 3). However, the amplitude of leakage signal 80 may be suchthat an eavesdropper could detect and amplify leakage signal 80, andthus eavesdrop on the phone conversation on telephone 30A.

Leakage signal 80 arises from the manner in which a plurality ofcommunication connections are coupled to a single communication device,such as the MVL transceiver 60. Each of the communication connectionsare physically coupled to each other by virtue of their connection tovarious electrical devices. For example, as illustrated in FIG. 3,subscriber loop 26A is physically coupled to subscriber loop 26D throughlow pass filter 36A, communication connection 62 and low pass filter36D. Because of the impedance characteristics associated with theelectrical devices which separate subscriber loop 26A and 26D,communication signals associated with telephone conversations onsubscriber loop 26A are typically attenuated such that leakage signalsassociated with telephone conversations on subscriber loop 26A will notsubstantially interfere with communications occurring on subscriber loop26D. One skilled in the art will appreciate that leakage signal 80 willhave some characteristics which are similar to the well known phenomenonof cross-talk. However, cross-talk is quite different from the leakagesignal 80. Cross-talk arises from the inductive or capacitive couplingbetween two communication connections which are substantially adjacentand parallel to each other. Thus, leakage signal 80 is not considered tobe a cross-talk phenomenon.

SUMMARY OF THE INVENTION

The eavesdropping prevention system and method in accordance with thepresent invention provides an improvement to a communicationenvironment, wherein the eavesdropping prevention system and methoddeters a potential third party eavesdropper from detecting a leakagesignal on a multiple channel communication system having a plurality ofcommunication connections coupled to a plurality of communicationdevices which are in communication with a common multiple channelequipment unit.

A first embodiment of the eavesdropping prevention system and method, aconnection sharing multipoint low pass filter with intelligenttermination, employs a high-pass filter which effectively blocks thelower frequency leakage signal 80 (FIG. 4B), as described hereinafterand as shown in FIGS. 5–9. The cut-off frequency of the leakage signal(LS) filter 84A–84D (FIG. 6) would be conveniently selected to fallbetween the upper range of the leakage signal 80 frequency,approximately 4 KHz, and the low-end frequency F3 of data signal 78(FIG. 4B). This first embodiment of the eavesdropping prevention systemand method includes a detect and terminate functions 86A–86D (FIG. 6)which detects service on the communication connection to which each oneof the LS filters 84A–84D are coupled to. The detect and terminatefunctions 86A–86D detects service on the communication connection toensure that each LS blocking splitter 82A–82D is coupled to anin-service communication connection. If the communication connectionbecomes out-of-service, such as when a customer discontinues servicewith the service provider, the detect and terminate functions 86A–86Dwill automatically de-couple the respective LS filter (84A–84D) from thecommunication connection so that the LS filter (84A–84D) cannotintroduce undesirable harmonics or impedance distortion into thecommunication system. In an alternative embodiment, the detect andterminate functions 86A–86D would insert an impedance matching element.

The LS blocking splitter eavesdropping prevention system and method canalso be conceptualized as providing one or more methods for blockingleakage signals and uncoupling connections in a communication system. Inaccordance with one method of the invention, the method may be broadlysummarized by the following steps: blocking a leakage signal, detectingservice on a communication connection, and uncoupling the communicationconnection from a filter when the communication connection is not inservice.

A second embodiment of the eavesdropping prevention system and method, aconnection sharing multipoint POTS splitter employing an amplifier-basedcoupler 146 (FIG. 10), is constructed with a nearly-zero impedance pathwhich effectively prevents the propagation of a leakage signal ontoother communication connections which are coupled to the same multiplevirtual connection (MVL) transceiver 60, or another communicationdevice, as described hereinafter and as shown in FIGS. 10–12. Leakagesignals are highly attenuated by the nearly-zero amplifier outputimpedance.

The amplifier-based coupler eavesdropping prevention system and methodcan also be conceptualized as providing one or more methods for shuntingleakage signals in a communication system. In accordance with one methodof the invention, the method may be broadly summarized by the followingsteps: coupling an amplifier having a low impedance characteristicbetween a communication connection and a communication device, andshunting at least one leakage signal originating on the communicationconnection over the low impedance amplifier thereby preventing theleakage signal from propagating to a second communication connectionhaving a higher impedance characteristic.

A third embodiment of the eavesdropping prevention system and method, aconnection sharing multipoint POTS splitter employing a mask signalgenerator, generates a mask signal 256 (FIG. 14) which is superimposedover leakage signal 80 such that the underlying leakage signal 80 cannotbe meaningfully detected and amplified, as described hereinafter and asshown in FIGS. 13–16. The amplitude of mask signal 256 is low enough soas not to interfere with the transmission of analog POTS signals 258(FIG. 14) over the communication connection on which the mask signal 256is superimposed. In one embodiment, the amplitude of the mask signal 256is large enough to exceed the amplitude of any anticipated leakagesignal 80 which may be manifested on the communication connection.Alternative embodiments of a mask signal are shown in FIGS. 16A–16C.

The mask signal generator eavesdropping prevention system and method canalso be conceptualized as providing one or more methods for generating amask signal which prevents meaningful detection and amplification of theleakage signal. In accordance with one method of the invention, themethod may be broadly summarized by the following steps: generating amask signal and transmitting the mask signal onto a communicationconnection.

A fourth embodiment of the eavesdropping prevention system and method, aconnection sharing multi-point transceiver employing a controllable lineselection unit, isolates a plurality of communication lines such thatthe underlying leakage signal cannot be meaningfully detected andamplified, as described hereinafter and as shown in FIGS. 17–22. Acontroller detects transitions between channels of a time-divisionmultiplexed communication signal and actuates a plurality of switchesresiding in the controllable line selection unit such that thetransceiver is coupled to selected communication connections on whichthe current channel is intended to be communicated over. Thecontrollable line selection unit controller detects transitions to thenext channel, and then actuates the switches such that the transceiveris coupled to a different communication connection for which the nextchannel is to be communicated over. The controllable line selectionunit, by selectively coupling the transceiver to selected communicationconnections, isolates the selected communication connections from theother communication connections thereby preventing the propagation of atleast one leakage signal.

The controllable line selection unit system and method can also beconceptualized as providing one or more methods for selectively couplinga transceiver to one of a plurality of communication connections. Inaccordance with one method of the invention, the method may be broadlysummarized by the following steps: detecting transitions betweenpredefined channels of a communication signal, actuating at least oneswitching device upon the detection of the transition so that atransceiver is coupled to a first communication connection, andactuating the switching device upon the detection of the next transitionso that the transceiver is coupled to a second communication connection.

Other systems, methods, features, and advantages of the eavesdroppingprevention system and method will be or become apparent to one withskill in the art upon examination of the following drawings and detaileddescription. It is intended that all such additional systems, methods,features, and advantages be included within this description, be withinthe scope of the eavesdropping prevention system and method, and beprotected by the accompanying claims for the eavesdropping preventionsystem and method.

BRIEF DESCRIPTION OF THE DRAWINGS

The eavesdropping prevention system and method, as defined in theclaims, can be better understood with reference to the followingdrawings. The components within the drawings are not necessarily toscale relative to each other, emphasis instead being placed on clearlyillustrating the principles of the eavesdropping prevention system andmethod.

FIG. 1 is a block diagram illustrating a conventional telephony system.

FIG. 2 is a block diagram illustrating a multiple channel digitalsubscriber loop (DSL) system communicating over a single subscriber loopto the central office of FIG. 1.

FIG. 3 is a block diagram illustrating four customer premises coupled toa central office via four separate subscriber loops, with each of thecustomer premises having a digital device communicating with a multiplevirtual connection (MVL) digital equipment unit located in the centraloffice.

FIG. 4A is a simplified graphical representation of the availablecommunication system frequency spectrum having an analog plain oldtelephony system (POTS) signal and a data signal, both signals beingcommunicated over subscriber loop 26A of FIG. 3.

FIG. 4B is a simplified graphical representation of the availablecommunication system spectrum having a leakage signal corresponding tothe analog POTS signal of FIG. 4A and a digital data signal, bothsignals being communicated over subscriber loop 26D of FIG. 3.

FIG. 5 is a block diagram illustrating a telephone system employing afirst embodiment of the present invention, the connection sharingmultipoint POTS splitter with intelligent termination which blocks theleakage signal of FIG. 4B.

FIG. 6 is a block diagram illustrating a more detailed view of theconnection sharing multipoint POTS splitter with intelligent terminationof FIG. 5.

FIG. 7 is a block diagram illustrating a more detailed view of apossible implementation of the LS filter functional component shown inFIG. 6.

FIG. 8 is a block diagram illustrating the telephone systems of FIGS. 5and 6 with the POTS switching unit de-coupled from the POTS splitterassociated with subscriber loop 26A de-coupled.

FIG. 9 is a block diagram illustrating a more detailed view of apossible implementation of the detect and terminate function of FIGS. 6and 7.

FIG. 10 is a block diagram illustrating a telephone system employingsecond embodiment of the present invention, an amplifier-based coupler.

FIG. 11 is a block diagram illustrating a more detailed view of thereceive connection selector shown in FIG. 10.

FIG. 12 is a block diagram illustrating an alternative embodiment of theamplifier-based coupler of FIG. 10.

FIG. 13A is a block diagram illustrating a telephone system employing athird embodiment of the present invention, a mask signal generator.

FIG. 13B is a block diagram illustrating an alternative configuration ofthe mask signal generator.

FIG. 14 is a graphical representation of the available communicationsignal frequency spectrum associated with subscriber loop 26D of FIGS.13A and/or 13B illustrating how the mask signal generated by the masksignal generator of FIGS. 13A and/or 13B effectively masks a leakagesignal.

FIG. 15 is a block diagram illustrating components employed in anembodiment of the mask signal generator of FIGS. 13A and/or 13B.

FIGS. 16A–16C are graphical representations of the availablecommunication frequency spectrum associated with subscriber loop 26D ofFIGS. 13A and/or 13B illustrating possible variations in the mask signalof FIG. 14.

FIG. 17 illustrates a controllable line selection unit coupled to oneline coupler.

FIG. 18 illustrates an exemplary controllable switch timing sequenceapplied to a four channel time-duplexed communication signal by thecontrollable line selection unit.

FIG. 19 illustrates selected components of a preferred embodiment of thecontrollable line selection unit coupled to four line couplers.

FIG. 20 illustrates selected components which may be employed in acontroller implemented as part of a controllable line selection unitshown in FIG. 17.

FIG. 21 is a flow chart illustrating the operation of the logic of FIG.20 as applied to a method for controlling switch out positions in acontrollable line selection unit of FIG. 17.

FIG. 22 illustrates an alternative embodiment of a controllable lineselection unit.

FIG. 23 illustrates another alternative embodiment of a controllableline selection unit. For convenience of illustration, elements among theseveral figures that are similar to each other may bear the samereference numerals. Such elements bearing the same reference numeralsmay be considered to be like elements, however, since these likenumeraled elements are incidental to the operation of the presentinvention which utilizes existing portions of a communication network,one skilled in the art will realize that like numeraled elements amongthe several figures need not be identical, as any variations of suchelements will not adversely affect the functioning and performance ofthe present invention. Furthermore, like elements that are like-numberedmay be described in detail only in the first instance of occurrence, andnot described in detail again when occurring in subsequent figures.

DETAILED DESCRIPTION OF THE INVENTION

A. Overview of the Eavesdropping Prevention System and Method

When a plurality of communication connections are coupled to a commoncommunication device, leakage signals associated with signals on one ofthe communication connections may propagate onto one or more of theother communication connections. Such a propagated signal is definedherein as a leakage signal. For convenience of describing the nature ofthe leakage signal and the eavesdropping prevention system and method ofthe present invention, a leakage signal 80 (FIG. 4B) and the presentinvention are described above in reference to a single multiple virtualline (MVL) transceiver 60 (FIG. 5) coupled to four subscriber loops26A–26D. One skilled in the art will appreciate that the presentinvention, and the associated effects of a leakage signal, may beequally applicable to other types of communication systems. Any suchapplication of an eavesdropping prevention system and method of thepresent invention, as described hereinafter, employed on such othercommunication systems are intended to be within the scope of thisdisclosure and be protected by the accompanying claims for the presentinvention.

In the absence of the eavesdropping prevention system and method of thepresent invention, a potential third-party eavesdropper having access tothe other communication connections might be able to detect the leakagesignal 80, and with appropriate amplification of the leakage signal 80,be able to eavesdrop on a telephone conversation occurring on one of theother communication connections. The eavesdropping prevention system andmethod will prevent, or at least make more difficult, the detection ofleakage signal 80.

Generally described, the present invention pertains to an eavesdroppingprevention system and method which prevents, or at least makes moredifficult, the detection of leakage signal 80. A first embodiment of theeavesdropping prevention system and method, the connection sharingmultipoint POTS splitter with intelligent termination, employs a leakagesignal (LS) filter which effectively blocks the lower frequency leakagesignal 80, thereby preventing the leakage signal from propagating toother communication connections which are coupled to a commoncommunication device such as, but not limited to, a multiple virtualconnection (MVL) digital equipment unit. Also included may be a detectand terminate function which detects service on the communicationconnection to which the LS filter is coupled to. The detect andterminate function automatically de-couples (terminates) the LS filterif the communication connection becomes out-of-service. The secondembodiment of the eavesdropping prevention system and method includes anamplifier-based coupler configured with a nearly-zero impedance path,which shunts the leakage signal away from the other communicationconnections. The third embodiment of the eavesdropping prevention systemand method includes a mask signal generator which generates a masksignal that is superimposed over leakage signal 80. The fourthembodiment of the eavesdropping prevention system and method includes acontrollable line selection unit which isolates the communicationconnection over which a signal is being communicated from othercommunication connections.

B. First Embodiment of the Eavesdropping Prevention System and Method: AConnection Sharing Multipoint POTS Splitter with Intelligent Termination

1. LS Blocking Splitter

FIG. 5 illustrates a portion of a plain old telephony system (POTS) 20″employing the first embodiment of the eavesdropping prevention systemand method, a connection sharing multipoint POTS splitter withintelligent termination, hereinafter referred to as the leakage signal(LS) blocking splitter for convenience. With this preferred embodimentof the LS blocking splitter system and method for eavesdroppingprevention, LS blocking splitter 82A may be disposed between a MVLtransceiver 60 and subscriber loop 26A such that a leakage signal 80associated with a telephone conversation occurring in customer premises24A cannot propagate from subscriber loop 26A onto connection 62. Asdescribed hereinabove, if leakage signal 80 (FIG. 4B) propagates ontoconnection 62 and then onto subscriber loops 26B, 26C and/or 26D,leakage signal 80 might be detectable on subscriber loops 26B, 26Cand/or 26D. LS blocking splitter 82A sufficiently attenuates a leakagesignal 80, as described hereinafter, to levels below the system noisefloor. That is, LS blocking splitter 82A blocks leakage signal 80 fromconnection 62, and thereby effectively blocks leakage signal 80 frombeing detected on subscriber loops 26B, 26C and/or 26D.

MVL transceiver 60 is used for convenience of illustration. MVLtransceiver 60 receives and transmits digital signals from/to aplurality of digital devices (not shown) residing in customer premises24A–24D over subscriber loops 26A–26D, respectively. The LS blockingsplitter system and method for eavesdropping prevention will workequally well with any similarly functioning communication device orother communication devices wherein a plurality of communicationconnections are coupled together such that leakage signals may propagateonto the commonly coupled communication connections. It is intended thatall such additional systems and communication devices employing the LSblocking splitter be included within the scope of this disclosure and beprotected by the accompanying claims for the LS blocking splitter.

Similarly, LS blocking splitter 82B will prevent a leakage signalassociated with a telephone conversation occurring in customer premises24B from propagating from subscriber loop 26B onto connection 62.Likewise, LS blocking splitter 82C and LS blocking splitter 82D preventleakage signals associated with telephone conversations occurring incustomer premises 24C and 24D, respectively, from propagating ontoconnection 62.

FIG. 6 illustrates in more detail the LS blocking splitter. LS blockingsplitter 82A is seen to have at least two functional components, an LSfilter functional component 84A and a detect and terminate functionalcomponent 86A. Similarly, LS blocking splitter 82D is shown to have atleast an LS filter functional component 84D and a detect and terminatefunctional component 86D. LS blocking splitter 82B (not shown) coupledto subscriber loop 26B and customer premises 24B (not shown), and LSblocking splitters 82C (not shown) coupled to subscriber loop 26C andcustomer premises 24C (not shown) would include the LS filter functionalcomponent and the detect and terminate functional component. Connectionsfor LS filter functional component 84A and detect and terminatefunctional component 86A residing within LS blocking splitter 82A areshown for convenience of illustration only in FIG. 6. A more detailedview of connections for one possible implementation of the LS filterfunctional component 84A and for one possible implementation of thedetect and terminate functional component 86A will be described indetail hereinafter.

2. LS Filter

FIG. 7 illustrates components that may be used in a possibleimplementation of the LS filter functional component 82A shown in FIG.6. LS filter 88A is shown to be coupled to MVL transceiver 60 viaconnections 62. Here, two connections 62 are shown coupled to LS filter88A. Also, two connections 62 are shown continuing to LS filters (notshown) associated with customer premises 24B–24D (not shown). Theseconnections 62 correspond to the single connection 62 of FIGS. 5 and 6.One skilled in the art will appreciate that in a telephony system 20″,connections shown in FIGS. 5 and 6 represent wire pairs, also known as aloop. Such connections may be properly shown as a single connection inFIGS. 5 and 6 for convenience of illustration.

LS filter 88A is shown coupled to the detect and terminate functionalcomponent 86A by connections 90. The detect and terminate functionalcomponent 86A is coupled to POTS splitter 36A via connections 92A. Theseconnections 92A correspond to the single connection 92A shown in FIGS. 5and 6, which are shown as a single connection for convenience ofillustration. Subscriber loop 26A is shown to have two conductors, thetip conductor and the ring conductor, as is well known in the art.Subscriber loop 26A is shown coupling into low pass filter 36A.Connections 94A couple the low pass filter 36A to the CO POTS switchingunit 28. For illustrative convenience, connections 94A and subscriberloop 26A are shown as single connections in FIGS. 5 and 6 and as wirepairs in FIG. 7. Connections 94A and subscriber loop 26A are configuredwithin the low pass filter 36A to provide the necessary connectivity, asis well known in the art.

LS filter 88A is a high-pass filter employing resistive, capacitive andinductor elements. Resistor elements R96, capacitor elements C98 andinductor element L100 are selected such that LS filter 88A will preventleakage signal 80 (FIG. 4B) from propagating from subscriber loop 26A,through the connections and components associated with LS blockingsplitter 82A, onto connections 62. The cut-off frequency associated withthe preferred embodiment of LS filter 88A would be selected to have afrequency between 4 kHz and frequencies F1 and/or F3 (FIGS. 4A and 4B).The LS filter 88A cut-off frequency is so selected to allow data signal74 and/or data signal 78 (FIGS. 4A and 4B) to propagate throughtelephony system 20″ and to block leakage signal 80. Actual values ofR96, R98 and L100 may be determined and implemented using well knowntechniques commonly employed in the art of filter design andconstruction. Any suitable cut-off frequency which blocks leakage signal80 without interfering with the propagation of data signal 74 may beselected for LS filter 88A without departing substantially from thespirit and principles of the LS blocking splitter 82A. Furthermore, LSfilters 88B–88D, may be constructed substantially identical to filter88A, or alternatively, LS filters 88B–88D may be constructed withdiffering cut-off frequencies or differing components and/orconfigurations such that the operation and functionality of the LSfilters 88A–88D effectively block leakage signals. It is intended thatall such variations in the construction of LS filter 88A, includingvariations in configuration and/or variations in the number and/or sizeof the resistive, capacitive and inductive elements, be within the scopeof this disclosure and be protected by the accompanying claims.

3. Detect and Terminate Function/Component Overview

The preferred embodiment of the LS blocking splitter includes anoptional detect and terminate functional component 86A, in FIG. 7, isshown to be coupled to LS filter 88A via connections 90 and tosubscriber loop 26A via connections 92A. These connections for thedetect and terminate functional component 86A with connections 90 and92A are shown for convenience of illustration only. Actual connectionsfor one implementation of the detect and terminate functional component86A will be described in detail hereinafter and shown in FIG. 9.

FIG. 8 illustrates a telephony system 20″ corresponding to the portionof the telephony system 20″ shown in FIG. 5. However, connection 94A hasbeen de-coupled between POTS switching unit 28 and the low pass filter36A, as indicated by the single dashed connection for illustrativeconvenience. Subscriber loop 26A may become de-coupled in any variety ofmanners (hereinafter referred to as an out-of-service condition). Suchan out-of-service condition might occur when the customer associatedwith customer premises 24A has discontinued service with the serviceprovider. For example, the customer may have vacated the customerpremises 24A or may have been de-coupled for failure to make payment tothe service provider. Alternatively, the service provider could effectan out-of-service condition by de-coupling at other convenientlocations, such as, but not limited to, within POTS switching unit 28and/or the low pass filter 36A.

In any of the above-described scenarios, or in similar situations,proper functioning of the MVL transceiver 60 or other communicationdevice may require the detection of the out-of-service condition andappropriate actuation of switches to de-couple LS blocking splitter 82Afrom subscriber loop 26A. The detection of the out-of-service conditionand the associated de-coupling of LS blocking splitter 82A is performedby the detect and terminate functional component 86A (FIG. 7). LSblocking splitters 82B–82D would also employ a detect and terminatefunctional component (not shown), similar to the detect and terminatefunctional component 86A, to identify out-of-service conditions on theirrespective subscriber loops 26B–26D. During an out-of-service condition,the impedance characteristics associated with the LS filter functionalcomponent and the impedance characteristics of the low pass filter 36A,subscriber loop 26A and any connected equipment at customer premises 24Amay be such that the communication of data signals by MVL transceiver60, equipment in customer premises 24B–24D, and/or other equipment (notshown) in CO 22 might be adversely affected by interference signalsgenerated by subscriber loop 26A, by low pass filter 36A, and/or byequipment residing in customer premises 24A. Therefore, the detect andterminate functional component (not shown) of LS blocking splitters82A–82D may be required to de-couple and reconfigure LS filters 88A–88D,respectively, in a manner described hereinafter.

FIG. 9 illustrates one possible implementation of components used in thepreferred embodiment of the detect and terminate functional component86A (see also FIGS. 6 and 7) residing in LS blocking splitter 82A.Components associated with the detect and terminate functional component86A include detector 110, switch controller 112, switches 114, switch116 and matching impedance element 118. Switches 114 are coupled to LSfilter 88A via connections 126, and are coupled to matching impedance188 via connections 128. Switch 116 is coupled to LS filter 88A viaconnections 130.

4. Detector Employed in the Detect and Terminate Functional Component

In the embodiment of the LS blocking splitter 82A shown in FIG. 9,detector 110 is shown to be detecting voltage across the connections 92Awhich couple LS blocking splitter 82A to subscriber loop 26A (see alsoFIGS. 5–8). In the preferred embodiment illustrated in FIG. 9, detector110 includes a high-input impedance instrumentation amplifier (IA) 120.In alternative embodiments of detector 110, voltage on one of theconnections 92A may be detected. Another alternative embodiment ofdetector 110 may sense current on one or both connections 92A. Yetanother alternative embodiment of detector 110 may detect voltage and/orcurrent on subscriber loop 26A. Such detector methods and apparatus arewell known and commonly employed in the arts of measuring electricalcurrent and voltage, and are not described in detail herein. Any suchvariations and/or modifications in the detector method employed in an LSblocking splitter may be employed without departing substantially fromthe spirit and principles of the present invention. Furthermore,detector methods and apparatus employed in the LS blocking splitter mayreside in convenient alternative locations, such as, but not limited to,other electrical equipment or in stand alone facilities, withoutadversely affecting the functionality of the LS blocking splitter. Anysuch alternative embodiments of the detector methods and apparatus soemployed are intended to be within the scope of this disclosure and beprotected by the accompanying claims for the LS blocking splitter systemand method.

5. Switch Controller Employed in the Detect and Terminate FunctionalComponent

In the embodiment of LS blocking splitter 82A illustrated in FIG. 9,detector 110 provides information corresponding to the detected voltageon connections 92A to switch controller 112 via control connection 121.Switch controller 112 actuates switches 114 via control connections 122,and switch 116 via control connection 124. The purpose of switches 114and switch 116 is to de-couple LS filter 88A and to couple matchingimpedance element 118.

In the normal operating state, where MVL transceiver 60 is incommunication with equipment residing at customer premises 24A (anin-service condition), LS filter 88A is coupled between MVL transceiver60 and subscriber loop 26A by the appropriate configuration of switches114 and switch 116. Signals transmitted from MVL transceiver 60 tocustomer premises 24A propagates over connections 62 through switches114, over connections 126 through LS filter 88A, over connections 130through switch 116, and then onto customer premises 24A via subscriberloop 26A. Data signals transmitted by equipment residing in customerpremises 24A are transmitted to MVL transceiver 60 over the same path(in reverse order).

When service to customer premises 24A is in the out-of-servicecondition, such as when connections 94A are opened to de-couple low passfilter 36A and POTS switching unit 28 (FIG. 8), detector 110 detectsthis out-of-service condition. For example, detector 110 as shown inFIG. 9 may be detecting DC voltage on one or both of the connections92A. The out-of-service condition here would be detected when DC voltagechanges to substantially zero (0) volts. Detector 110 provides anindication of the out-of-service condition to switch controller 112 suchthat switches 114 and 116 are actuated to reconfigure the LS blockingsplitter 82A by de-coupling LS filter 88A and by coupling matchingimpedance element 118. Thus, switches 114 couple matching impedanceelement 118 to connections 62 via connections 128.

6. Matching Impedance Employed in the Detect and Terminate FunctionalComponent

Matching impedance element 118 corresponds to the impedance seen fromMVL transceiver 60 out to subscriber loop 26A. Matching impedanceelement 118 has resistive, capacitive and/or inductive components sizedand configured to approximate the impedance characteristics of thesystem seen by MVL transceiver 60 when looking out to subscriber loop26A. This matching impedance would approximately match the impedancecharacteristics of LS blocking splitter 82A so as to maintain a balancedimpedance system. Such a balanced impedance system may be desirable toensure acceptable performance of MVL transceiver 60 or othercommunication devices. One skilled in the art will appreciate thatdetermining transmission system impedance characteristics seen by MVLtransceiver 60 is well known in the art, and therefore, is not describedin detail herein. Furthermore, one skilled in the art will appreciatethat the determination, selection and configuration of impedancecomponents (resistive, capacitive and/or inductive) employed within thematching impedance element 118, may be determined and implemented usingwell known techniques commonly employed in the art of impedancematching. Therefore, the easily determined elements employed in matchingimpedance element 118 and their numerous configurations are notdescribed in detail herein. The numerous apparatus and methods ofconstructing matching impedance element 118 may be employed in theabove-described embodiment of the LS blocking splitter 82A withoutdeparting substantially from the spirit and principles of the LSblocking splitter. It is intended that all such systems, methods andconfigurations of matching impedance element 118 be included hereinwithin the scope of this disclosure and be protected by the accompanyingclaims for the LS blocking splitter.

7. Alternative Embodiments of an LS Blocking Splitter

An alternative embodiment of the LS blocking splitter could beincorporated as a functioning component of a stand alone POTS splitter.That is, referring to FIG. 5, low pass filter 36A and LS blockingsplitter 82A could be integrated into a single POTS splitter.

Another alternative embodiment of the LS blocking splitter 82A (asdescribed by referencing elements shown in FIG. 9 for convenience) maynot require matching impedance element 118. In such an alternativeembodiment, switches 114 should be actuated to de-couple LS filter 88Afrom connection 62. Another alternative embodiment of the detect andterminate function 86A may not employ switches 114, but may merelyactuate switch 116 to isolate MVL transceiver 60 and LS blockingsplitter 82A from subscriber loop 26A.

Another alternative embodiment may not use switches 114 to de-couple LSfilter 88A, but rather switch in matching impedance element 118 inparallel with LS filter 88A such that the desired impedancecharacteristic seen by MVL transceiver 60 is achieved. Anotheralternative embodiment of the detect and terminate function 86A could beconfigured to switch matching impedance element 118 in series or inparallel with LS filter 82A, thereby achieving the desired impedancecharacteristics. Any such implementations of the LS blocking splitters82A–82D, and LS blocking splitters employed in alternative embodimentsof the present invention, are intended to be within the scope of thisdisclosure and be protected by the accompanying claims.

Switch controller 112 may be implemented as hardware or a combination ofhardware and firmware. When implemented as hardware, switch controller112 can be constructed of any of the commonly employed technologies inthe well known art of controlling switches. An alternative embodiment ofthe switch controller 112 may be implemented as firmware, software orother computer-readable medium stored in a memory (not shown) that isexecuted by a suitable microprocessor (not shown) residing in switchcontroller 112 or residing in another convenient location and incommunication with switch controller 112. Software instructionsassociated with a program which implements the detect and terminatefunction, which each comprise an ordered listing of executableinstructions for implementing logical functions, can be embodied in anycomputer-readable medium for use by or in connection with an instructionexecution system, apparatus, or device, such as a computer-based system,processor-containing system, or other system that can fetch theinstructions from the instruction execution system, apparatus, or deviceand execute the instructions. Any such implementations of the switchcontroller 112 are intended to be within the scope of this disclosureand be protected by the accompanying claims for the LS blockingsplitter.

In the context of this document, a “computer-readable medium” can be anymeans that can contain, store, communicate, propagate, or transport theprogram for use by or in connection with the instruction executionsystem, apparatus, or device. The computer readable medium can be, forexample but not limited to, an electronic, magnetic, optical,electromagnetic, infrared, or semiconductor system, apparatus, device,or propagation medium. More specific examples (a non-exhaustive list) ofthe computer-readable medium would include the following: an electricalconnection (electronic) having one or more wires, a portable computerdiskette (magnetic), a random access memory (RAM) (electronic), aread-only memory (ROM) (electronic), an erasable programmable read-onlymemory (EPROM or Flash memory) (electronic), an optical fiber (optical),and a portable compact disc read-only memory (CDROM) (optical). Notethat the computer-readable medium could even be paper or anothersuitable medium upon which the program is printed, as the program can beelectronically captured, via for instance optical scanning of the paperor other medium, then compiled, interpreted or otherwise processed in asuitable manner, if necessary, and then stored in a computer memory.

The switching functions performed by switches 114 and switch 116, ascontrolled by switch controller 112, may be implemented with any type ofelectronic, solid state or firmware type switching device or meanscommonly employed in the art. Such controlled switches 114 and switch116, in the above-described embodiment or in alternative embodiments,could be implemented by a combination of software and firmware usingcomponents and methods commonly employed in the art of switchingelectrical devices. It is intended that all such implementation ofswitches 114 and switch 116, and their associated control means, beincluded herein within the scope of this disclosure and be protected bythe accompanying claims for the LS blocking splitter.

C. Second Embodiment of the Eavesdropping Prevention System and Method:A Connection Sharing Multipoint POTS Splitter with an Amplifier-BasedCoupler

1. Amplifier-Based Coupler Overview

FIG. 10 illustrates a portion of a telephony system 20′″, whichcorresponds to telephony system 20″ (FIG. 3), employing the secondembodiment of the eavesdropping prevention system and method, aconnection sharing multipoint POTS splitter with an amplifier-basedcoupler, hereinafter referred to as the amplifier-based coupler forconvenience. Amplifier-based coupler 146 couples MVL transceiver 60 witha plurality of subscriber loops 26A–26C. For convenience ofillustration, the amplifier-based coupler 146 couples four subscriberloops 26A–26D to MVL transceiver 60. However, the amplifier-basedcoupler 146 could be configured to couple two subscriber loops, threesubscriber loops, or more than four subscriber loops, to the MVLtransceiver 60. MVL transceiver 60 is used for convenience ofillustration. The amplifier-based coupler 146 will work equally wellwith any similarly functioning communication device or othercommunication devices wherein a plurality of communication connectionsare coupled together such that leakage signals may propagate unto ontothe commonly coupled communication connections. It is intended that allsuch additional systems and communication devices employing theamplifier-based coupler 146 be included within the scope of thisdisclosure and be protected by the accompanying claims for theamplifier-based coupler 146.

2. MVL Transceiver

For convenience of illustration and to facilitate the disclosure of thefunctionality and operation of the amplifier-based coupler 146, theamplifier-based coupler 146 is shown to be coupled with MVL transceiver60. MVL transceiver 60 has at least the well known components of atransmitter 140 and a receiver 144. Transmitter 140, as employedaccording to FIG. 10, is a balanced differential voltage signal source.Operation of the transmitter 140 and receiver 144 are not described indetail herein other than to the extent necessary to understand theoperation and functioning of these components as part of an MVLtransceiver 60, employing the amplifier-based coupler system and methodof the present invention. Accordingly, the amplifiers 152 and 154amplify a voltage signal received from transmitter 140.

One skilled in the art will realize that MVL transceiver 60 and othersimilarly functioning communication devices, may have the transmitter140 and/or receiver 144 connected in a different order and/or mannerthan shown in FIG. 10, or may not include both the transmitter 140 andreceiver 144 as shown in FIG. 10, or may include additional componentsconnected in some other manner with the transmitter 140 and/or receiver144. For example, transmitter 140 could be a signal current source, andamplifiers 152 and 154 would then be configured to amplify a currentsignal received from transmitter 140. Also, the amplifier-based couplercould be employed to couple other types of communication devices to aplurality of communication connections. Any of the above-describedvariations may be made without departing substantially from the spiritand principles of the amplifier-based coupler 146 system and method, andas such, are not pertinent to an explanation of the operation of theamplified-based coupler and are not explained in detail herein. Any suchvariations in a MVL transceiver 60, or other communication device,employing the amplifier-based coupler 146 system and method are intendedto be within the scope of this disclosure and be protected by theaccompanying claims for the amplifier-based coupler 146.

MVL transceiver 60 is coupled to the preferred embodiment ofamplifier-based coupler 146 via two connections 148 and 150. Connections148 and 150 provide the path for the positive transmit signal (Tx+) andthe negative transmit signal (Tx−), respectively, generated by thetransmitter 140. Tx+ is received by a first amplifier 152 via connection148. The Tx− signal is received by a second amplifier 154 via connection150. Amplifier 152 amplifies the Tx+ signal to an appropriate powerlevel and transmits the amplified Tx+ signal onto connection 156.Amplifier 154 amplifies the Tx− signal to an appropriate power level andtransmits the amplified Tx− signal onto connection 164.

3. Line Couplers

For illustrative convenience and to disclose the functionality andoperation of amplifier-based coupler 146, amplifier-based coupler 146 isshown to be coupled to line couplers 158A–158D. Connection 156 couplesto line couplers 158A–158D. Line coupler 158A, also known as atransformer, provides magnetic coupling to subscriber loop 26A such thatthe amplified Tx+ signal on connection 156 is induced onto the tipconductor of subscriber loop 26A through inductor L160 and inductorL162. Similarly, the Tx− signal is amplified by amplifier 154 and outputonto connection 164. The amplified Tx− signal on connection 164 isinduced onto the ring conductor of subscriber loop 26A through inductorL160 and inductor L166. As is well known in the art, line coupler 158Atypically includes a resistance R168, an iron core 170 to facilitatemagnetic coupling between inductor L160 and inductor L162 and forcoupling between inductor L160 and inductor L166, and a direct current(DC) blocking capacitor C172. Detailed operation of these individualcomponents of line couplers 158A–158D are well known in the art, and assuch, are not described in detail herein other than to the extentnecessary to understand the operation and functioning of thesecomponents as related to the functioning and operation of theamplifier-based coupler 146. One skilled in the art will realize thatline couplers 158A–158D, other transformer-based driver systems, orother similarly performing circuits, may have the components shown inFIG. 10 connected in a different order and manner than shown in FIG. 10,or may not include all of the components shown in FIG. 10, or mayinclude additional components connected in some manner with thecomponents of the line couplers 158A–158D shown in FIG. 10. For example,a Norton based equivalent circuit employing current output amplifiersmay be used to provide the necessary coupling between the MVLtransceiver 60 and subscriber loops 26A–26D. Any such variations in aline coupler or similarly performing circuit which employs theamplifier-based coupler 146 system and method may be made withoutdeparting substantially from the spirit and principles of theamplifier-based coupler 146, and as such, are intended to be within thescope of this disclosure and be protected by the accompanying claims forthe amplifier-based coupler 146.

Line coupler 158B is shown to couple connections 156 and 164 tosubscriber loop 26B. For convenience of illustration, a detailed diagramof line coupler 158B showing internal components similar to thecomponents of line coupler 158A is not shown. Line coupler 158B, whencoupled to subscriber loop 26B and connections 156 and 164, would becoupled in a similar manner and have like components as the line coupler158A. Likewise, line coupler 158C couples connections 156 and 164 tosubscriber loop 26C and line coupler 158D couples connections 156 and164 to subscriber loop 26D. Thus, the amplified Tx+ signal on connection156 is transmitted to the tip conductor of each of the subscriber loops26A–26D, and the amplified Tx− signal on connection 164 is transmittedto the ring conductor of each of the subscriber loops 26A–26D.Subscriber loops 26A–26D provide the transmission path to customerpremises 24A–24D (FIG. 3). Thus, digital devices 38A–38D (FIG. 3) areable to receive data signals (Tx+, Tx−) transmitted by the MVLtransceiver 60 transmitter 140.

Alternative embodiments of a line coupler may incorporate inductors L162and L166 into a POTS splitter (not shown). Or, a line coupler may beincluded as part of a POTS splitter (not shown) or similarly functioningdevice. One skilled in the art will appreciate that these alternativeembodiments of the line coupler may be made without departingsubstantially from the spirit and principles of the amplifier-basedcoupler 146, and as such, are intended to be within the scope of thisdisclosure and be protected by the accompanying claims for theamplifier-based coupler 146.

Data signals transmitted from the customer premises 24A–24D arrive atline couplers 158A–158D, respectively. When a signal is transmitted bydigital device 38A from customer premises 24A (FIG. 3), a positivereceived signal (Rx+) is delivered over the tip conductor of subscriberloop 26A to line coupler 158A. The magnetic coupling between inductorL162 and inductor L160 allows the Rx+ signal to be transmitted toconnection LA+. In one system employing MVL transceiver 60, theconnection LA+ is coupled to a receive line selector 174. The Rx+ signalassociated with customer premises 24A (FIG. 3) is then passed to MVLtransceiver 60 receiver 144 over connection 176 in a manner describedhereinafter. Similarly, a negative receive signal (Rx−) is deliveredover the ring conductor of subscriber loop 26A to the line coupler 158A.Rx− is transmitted to connection LA− through the magnetic couplingbetween inductor L166 and inductor L160. Receive line selector 174 thenpasses the Rx− signal to MVL transceiver 60 receiver 144 over connection178 in a manner described hereinafter. In a similar manner, linecouplers 158B–158D receive Rx+ and Rx− signals from customer premises24B–24D, respectively, and deliver the Rx+ and Rx− signals to receiver144.

4. Operation of the Amplifier-Based Coupler on Leakage Signals

The output of the first amplifier 152 is coupled to the negative inputof amplifier 152 via connection 180 such that amplifier 152 is coupledin a negative feedback amplifier configuration. Similarly, the output ofamplifier 154 is coupled to the negative input of amplifier 154 viaconnection 182 such that amplifier 154 is configured as a negativefeedback amplifier. The above-described configuration of amplifiers 152and 154 as negative feedback amplifiers creates a nearly-zero amplifieroutput impedance path between connections 156 and 164, respectively. Forexample, a leakage signal 80 (FIG. 4B) associated with a POTS telephoneconversation originating on subscriber loop 26A will be significantlyattenuated by the nearly-zero output impedance of amplifiers 152 and154. Thus, a leakage signal originating on subscriber loop 26A will notsubstantially appear on subscriber loops 26B, 26C and/or 26D.

5. Amplifier-Based Coupler Amplifier

One skilled in the art should appreciate that the size and/or powerrating of amplifiers 152 and 154 should be based upon the needs of theMVL transceiver 60 which are required to operate in telephony system20″. Amplifiers 152 and 154 may be conveniently selected from aplurality of standardized parts to economically facilitate manufacturingand assembly. Or, amplifiers 152 and 154 may be specially fabricatedamplifiers or similar electrical devices which perform substantially thesame functionality of amplifiers 152 and 154. As mentioned above,amplifiers 152 and 154 may be configured to amplify a voltage signalreceived from transmitter 140, or configured to amplify a current signalreceived from transmitter 140. Such components may be used in anamplifier-based coupler so long as the above-described nearly-zeroimpedance path is provided. Such variations in the amplifier-basedcoupler 146 may be implemented without departing substantially from thespirit and principles of the present invention. All such modificationsand variations of an amplifier-based coupler 146 are intended to beincluded herein within the scope of this disclosure and be protected bythe accompanying claims for the amplifier-based coupler 146.

The embodiment of the amplifier-based coupler 146 shown in FIG. 10,illustrates the amplifier-based coupler 146 being a separate stand-alonecomponent. Alternative embodiments of the amplifier-based coupler 146may be incorporated into MVL transceiver 60 as an integral component.Furthermore, line couplers 158A–158D are shown to be stand-alonecomponents. An alternative embodiment of the amplifier-based coupler 146could combine the elements of the amplifier-based coupler 146 and linecouplers 158A–158D into a single stand-alone unit or into an integralpart of MVL transceiver 60. Any such alternative embodiments of theamplifier-based coupler 146 may be implemented as described abovewithout departing substantially from the spirit and principles of thepresent invention. All such modifications and variations are intended tobe included herein within the scope of this disclosure and be protectedby the accompanying claims for the amplifier-based coupler 146.

6. Receive Line Selector

FIG. 11 illustrates one possible configuration of the receive connectionselector 174. For convenience of illustration, the line couplers158A–158D (not shown) would be coupled to receive connection selector174 at connection points LA+ to LD+, and LA− to LD−, in a mannerillustrated for line couplers 158A and 158D in FIG. 10. Thus, thepreferred embodiment of the amplifier-based coupler 146 is alsoeffectively coupled to receive connection selector 174. A detaileddescription of the operation of the receive connection selector 174 andthe associated components within receive connection selector 174 is notprovided herein other than to the extent necessary to understand theoperation and functioning of the receive connection selector 174 and theassociated components with respect to the operation and functioning ofthe amplifier-based coupler 146. One skilled in the art should realizethat receive connection selector 174, or a similar device, may havecomponents configured differently than shown in FIG. 11, may not includeall of the components shown in FIG. 11, or may include additionalcomponents connected in some other manner with the components shown inFIG. 11. Furthermore, alternative embodiments of amplifier-based coupler146 (FIG. 10) may be able to perform and operate satisfactorily in theabsence of a receive connection selector 174. For example, receiveconnection selector 174 may be omitted, and connection impedances mightbe employed to create a higher impedance path which prevents a leakagesignal 80 (FIG. 4B) from propagating from one communication connectionto another communication connection. Any such variations in a receiveconnection selector 174 or similar device which is employed along withan amplified-based coupler 146 are intended to be within the scope ofthis disclosure.

Receive connection selector 174, as shown in FIG. 11, has a firstamplifier 190 and a second amplifier 192 such that a positive receivesignal from connections LA+, LB+, LC+ and/or LD+ are amplified byamplifier 190 such that the output of amplifier 190 provides anamplified signal Rx+ that is of the appropriate amplitude which may bedetected by MVL transceiver 60 receiver 144. Likewise, a negativereceive signal from connections LA−, LB−, LC− and/or LD− are amplifiedby amplifier 192 and output via connection 196 to the MVL transceiverreceiver 144. Other resistive, capacitive and inductive components areincluded in receiver connection selector 174 (R198, C200, R202).

A selector 204 is coupled to amplifier 190. Selector 204 selects theappropriate connection LA+, LB+, LC+ or LD+ such that the amplified Rx+signals from customer premises 24A–24D (FIG. 3) may be properly detectedand transmitted to receiver 144 (FIG. 10). Similarly, selector 206 iscoupled to amplifier 192. Selector 206, in a manner similar to selector204, selects the appropriate connection LA−, LB−, LC− or LD− such thatthe Rx− signal can be transmitted to receiver 144 (FIG. 10). Theoperation and control of selectors 204 and 206, which is determinedbased upon the particular signal modulation scheme employed by the MVLtransceiver 60 (FIG. 10), is well understood in the art and is notdescribed in detail herein except to the extent necessary to understandthe operation and functioning of the amplifier-based coupler 146. Thatis, the nearly-zero impedance path associated with the amplifier-basedcoupler 146 effectively shunts the leakage signal 80, substantiallypreventing leakage signals from propagating onto other subscriber loops.

With respect to the operation of the receive line selector 174, theimpedances associated with switches 204 and 206, and with R202, are suchthat a leakage signal 80 (FIG. 4B) would not be transmitted from onesubscriber loop to another subscriber loop. Rather, any leakage signal80 would be substantially attenuated by the above-mentioned nearly-zeroimpedance path associated with the amplifier-based coupler 146

7. Amplifier-Based Coupler First Alternative Embodiment

FIG. 12 illustrates an alternative embodiment of an amplifier-basedcoupler system and method employing two amplifier-based couplers 210 and220. Amplifier-based coupler 210 employs a first amplifier 214 and asecond amplifier 216 which amplify the Tx+ signal and Tx− signal,respectively, received from the transmitter 140 residing in MVLtransceiver 60. The amplified Tx+ and Tx− signals from amplifier-basedcoupler 210 are transmitted to line coupler 218A and line coupler 218Bsuch that the Tx+ signal and Tx− signal are transmitted to customerpremises 24A and 24B over subscriber loops 26A and subscriber loop 26B,respectively. Elements within line coupler 218A that are similar to theelements in line coupler 158A (FIG. 10) bear the same reference numeralsas the elements of line coupler 158A. These like numeraled elements bearthe same reference numerals for convenience of illustration andexplanation, and may be considered to be like elements. However, sincethese like numeraled elements are incidental to the operation of theamplifier-based coupler 210, one skilled in the art should realize thatthe elements in line coupler 218A and line coupler 158A (FIG. 10) neednot be identical, as any variations of such elements will not adverselyaffect the functioning and performance of the amplifier-based coupler210.

Similar to the amplifier-based coupler 146 (FIG. 10), the amplifiers 214and 216 of the amplifier-based coupler 210 are configured as negativefeedback amplifiers. Thus, a nearly-zero impedance path through theamplifier-based coupler 210 is present such that leakage signals willnot be transmitted from subscriber loop 26A to subscriber loop 26B, orbe transmitted to subscriber loop 26B to subscriber loop 26A. Oneskilled in the art will appreciate that the amplifier-based coupler 210embodiment differs primarily from the amplifier-based coupler 146 (FIG.10) in that amplifiers 214 and 216 are driving communication signals(Tx+ and Tx−) onto only two subscriber loops (26A and 26B).

A second amplifier-based coupler 220, employing a first amplifier 222and a second amplifier 224, amplifies and transmits signals Tx+ and Tx−to subscriber loops 26C and 26D through line couplers 226C and 226D,respectively, in a like-manner described above for amplifier-basedcoupler 210. Thus, leakage signals from subscriber loop 26C do notpropagate onto subscriber loop 26D, or leakage signals from subscriberloop 26D do not propagate onto subscriber loop 26C, because the leakagesignal passes over the nearly-zero impedance path associated withamplifiers 222 and 224 of the amplifier-based coupler 220.

Leakage signals associated with POTS conversations at customer premises24A–26D will be significantly attenuated by the nearly-zero outputimpedance of amplifiers 214, 216, 222 and 224. Thus, leakage signalswill not substantially appear on other subscriber loops.

Line couplers 218A, 218B, 226C and 226D are coupled to receiveconnection selector 174. The operation and functionality of receiveconnection selector 174 is described above in association with FIG. 11and is not described in detail again. With the embodiment of theamplifier-based coupler system and method shown in FIG. 12, theimpedances (not shown) associated with the receive connection selector174 prevent the passage of a leakage signal from one subscriber loop toanother subscriber loop in the manner described hereinabove.

8. Amplifier-Based Coupler Alternative Embodiments

One skilled in the art will appreciate that any number of subscriberloops may be coupled to MVL transceiver 60 or similarly functioningcommunication device. The maximum number of subscriber loops beingcoupled is determined by the signal power requirements of thecommunication system and the signal multiplexing technology employed bythe MVL transceiver 60 or similarly coupled communication device.Similarly, amplifier-based couplers constructed in accordance with thesystem and method of the present invention may be coupled to any numberof subscriber loop line couplers. The number of subscriber loops coupledto a single amplifier-based coupler would be determined based upon theamplification capacity of the amplifiers employed in the amplifier-basedcoupler and the signal power requirements. Furthermore, anamplifier-based coupler employing the system and method of the presentinvention might employ one amplifier, or more than two amplifiers, whichare configured to couple MVL transceiver 60 or another communicationdevice to a communication connection such as, but not limited to, asubscriber loop.

In some applications, line couplers may be incorporated into otherdevices, or may not be required at all. In communication systems inwhich a line coupler is not employed, an impedance may be added tocreate a higher impedance path such that a leakage signal does notpropagate onto the communication connection. Alternatively, acommunication system not employing line couplers may have communicationconnections having sufficiently high inherent impedance such that aleakage signal will not be detectable on the communication connection.In these alternative embodiments, the amplifier-based coupler would beconfigured to have a nearly-zero impedance path such that a leakagesignal will be substantially attenuated by the amplifier-based couplerand not pass onto the communication connections.

Any such variations and modifications of an amplifier-based coupler inaccordance with the system and method of the present invention, may beimplemented without departing substantially from the spirit andprinciples of the amplifier-based coupler. Any such alternativeembodiments of an amplifier-based coupler system and method are intendedto be within the scope of this disclosure and be protected by theaccompanying claims for the amplifier-based coupler.

D. Third Embodiment of the Eavesdropping Prevention System and Method: AConnection Sharing Multipoint POTS Splitter with a Mask Signal Generator

1. Mask Signal Generator Overview

FIGS. 13A and 13B illustrate a portion of a telephony system 20″ (seealso FIG. 3) employing the third embodiment of the eavesdroppingprevention system and method, a connection sharing multipoint POTSsplitter with a mask signal generator, hereinafter referred to as a masksignal generator for convenience. With this preferred embodiment inaccordance with FIG. 13A, a mask signal generator 250 is disposed suchthat a mask signal 256 (FIG. 14), as described hereinafter, istransmitted onto connections 62A–62D via connection 252. Alternatively,as shown in FIG. 13B, mask signal 256 (FIG. 14) is transmitted ontosubscriber loops 26A–26D via connections 254A–254D, respectively.

Elements in FIGS. 13A and 13B that are similar to elements in FIGS. 1–4bear the same reference numerals. Such elements having the samereference numerals in FIGS. 1–3, 13A and 13B may be considered to belike elements. However, since these like numeraled elements areincidental to the operation of the mask signal generator 250 whichutilizes existing portions of telephony system 20″, one skilled in theart should realize that elements in FIGS. 1–3, 13A and 13B need not beidentical, as any variations of such elements will not adversely effectthe functioning and performance of the mask signal generator 250 asdescribed hereinafter. Therefore, like elements which are like-numberedwill not be described again in detail. MVL transceiver 60 is used forconvenience of illustration. The mask signal generator 250 will workequally well with any similarly functioning communication device orother communication devices wherein a plurality of communicationconnections are coupled together such that leakage signals may propagateonto the commonly coupled communication connections. It is intended thatall such additional systems and communication devices employing the masksignal generator 250 be included within the scope of this disclosure andbe protected by the accompanying claims for the mask signal generator250.

2. Mask Signal Generator

As shown in FIG. 13A, mask signal generator 250 generates a mask signal256 (FIG. 14) which is transmitted onto connections 62A–62D viaconnection 252. FIG. 13B shows an alternative embodiment of the masksignal generator 250 in that the mask signal 256 (FIG. 14) istransmitted onto subscriber loops 26A–26D directly via connections254A–254D, respectively.

FIG. 14 is a simplified illustrative example of a mask signal 256,generated by the mask signal generator 250 (FIGS. 13A and 13B), whichhas been transmitted onto connections 62A–62D (FIG. 13A) via connection252 (FIG. 13A), or, which has been transmitted onto subscriber loops26A–26D (FIG. 13B) via connections 254A–254D, respectively (FIG. 13B).FIG. 14 illustrates the available communication system frequencyspectrum 76 on subscriber loop 26D (FIGS. 13A and 13B). This FIG. 14corresponds to the available communication system frequency spectrum 76for subscriber loop 26D (see also FIG. 4B). Elements in FIG. 14 that aresimilar to those in FIG. 4B bear the same reference numerals. Suchelements having the same reference numerals in FIGS. 4B and 14 may beconsidered to be like elements, however, since these like numeraledelements are incidental to an explanation of the operation of the masksignal 256, one skilled in the art should realize that the elements inFIGS. 4B and 14 need not be identical, as any variations of suchelements will not adversely affect the functioning and performance ofthis third embodiment of the eavesdropping prevention system and method,the mask signal generator 250 (FIGS. 13A and 13B). Therefore, likeelements which are like-numbered will not be described again in detail.

A leakage signal 80, represented as a bold-dashed line in FIG. 14 inthis simplified illustrative example, is associated with an analog POTSsignal communicated over subscriber loop 26A from a person usingtelephone 30A (FIGS. 13A and 13B). A portion of the analog POTS signal(not shown) transmitted over subscriber loop 26A, propagates ontosubscriber loop 26D, thereby creating leakage signal 80 in a manner asdescribed hereinabove.

Additionally, a POTS signal 258 is shown in FIG. 14. POTS signal 258corresponds to a telephone conversation by a person at customer premises24D who is using telephone 30D (FIGS. 13A and 13B). POTS signal 258 istransmitted over subscriber loop 26D (FIGS. 13A and 13B). As illustratedin FIG. 14, the amplitude of leakage signal 80 is significantly lessthan the amplitude of POTS signal 258 such that leakage signal 80 doesnot interfere significantly with POTS signal 258. That is, leakagesignal 80 should not significantly interfere with telephoneconversations on subscriber loop 26D (FIGS. 13A and 13B). However, asdescribed hereinabove, the amplitude of leakage signal 80 may be suchthat an eavesdropper might detect and amplify leakage signal 80, andthus, eavesdrop on the phone conversation from a person talking ontelephone 30D.

Also shown in the simplified illustrative example of FIG. 14 is datasignal 78. As described hereinabove, data signal 78 is the signaltransmitted/received by PC 40D over subscriber loop 26D (FIGS. 13A and13B). Data signal 78 occupies a portion of the available frequencyspectrum from a frequency of F3 to F4, and thus, is seen to occupy aseparate portion of the available communication system frequencyspectrum 76 than the portion of the frequency spectrum utilized by POTSsignal 258 (and also leakage signal 80).

Mask signal generator 250 (FIGS. 13A and 13B) generates mask signal 256and transmits the mask signal 256 onto connection 62 (FIG. 13A) ordirectly onto subscriber loops 26A–26D (FIG. 13B). In the preferredembodiment, the amplitude of mask signal 256 is pre-determined such thatthe amplitude of mask signal 256 is greater than or at least equal toleakage signal 80. The amplitude of mask signal 256 should notsubstantially exceed the noise floor level of POTS signals at CO 22.Also, the frequency range of mask signal 256 is predefined tosubstantially correspond to the frequency range of leakage signal 80. Inthe preferred embodiment, the frequency range of mask signal 256 hasbeen predefined to be from 0 KHz to a frequency substantially equal toor greater than the 4 KHz upper frequency bandwidth of a typical analogPOTS signal, such as, POTS signal 258. Thus, one skilled in the artwould appreciate that a potential eavesdropper having access tosubscriber loop 26D, or customer premises 24D, or CO 22, would not beable to detect and amplify leakage signal 80. That is, mask signal 256effectively masks over leakage signal 80 such that leakage signal 80cannot be detected and amplified by the potential eavesdropper.

Mask signal 256, as shown in FIG. 14, is shown to be a constantamplitude noise signal. Mask signal 256 is illustrated as shown in FIG.14 for illustrative convenience and to facilitate an explanation of theeffect of mask signal 256 on the detectability of leakage signal 80. Oneskilled in the art will appreciate that mask signal 256 may be of anysuitable signal type which interferes with the detection of leakagesignal 80. One non-limiting example of mask signal 256 would be aconstant amplitude, white-noise signal.

3. Mask Signal Generator Control and Operation

FIG. 15 is a simplified illustrative block diagram of a preferredembodiment of the mask signal generator 250 (see also FIGS. 13A and13B). Components of the mask signal generator 250 includes at least asignal generator 260, a processor 262 and an interface 264. As is wellknown in the art, signal generators and signal generator control systemstypically contain many individual components aggregated together.However, these other associated elements are not relevant to anexplanation of the mask signal generator 250, and as such, only thosecomponents relevant to the functioning of the mask signal generator 250of the present invention are described herein. Processor 262 controlsthe signal generator 260 via connection 266. Logic 269 which controlsprocessor 262 may be provided by a user through interface 264 viaconnections 267 and 268 and stored in memory 270 via connection 272. Auser may be in communication with user interface 264 via connection 272to provide logic 269 that controls processor 262. In the preferredembodiment of mask signal generator 250, a user may specify the desiredamplitude and frequency characteristics of the mask signal 256 (FIG. 14)which is to be transmitted into connection 62 (FIG. 13A) or directlyonto subscriber loops 26A–26D (FIG. 13B).

The signal generator 260 illustrated in FIG. 15 generates mask signal256 onto two connections 252 through a high impedance source (to preventloading of connection pair 252). Resistors 261, in series with signalgenerator 260 providing a voltage-based mask signal 256 (FIG. 14), isone method of creating a high source impedance. Alternatively, signalgenerator 260 could be a current source, in which case resistors 261 maynot be necessary. The two connections 252 of FIG. 15 correspond to thesingle connection 252 of FIG. 13A, which is shown as a single connectionin FIG. 13A for convenience of illustration. As mentioned hereinabove,one skilled in the art should realize that typical telephony systems aretwo conductor systems which may be equivalently represented by twoconnections or by a single (pair) connection, depending on the natureand purpose of the block diagram illustration employed. Alternatively,connections 252 in FIG. 15 could have been labeled with referencenumerals 254A–254D to correspond with the four connections 254A–254D ofFIG. 13B which couple the mask signal generator 250 directly tosubscriber loops 26A–26D.

4. Alternative Embodiments Employing a Detector

FIG. 15 illustrates the use of a detector 276 with mask signal generator250. Detector 276 could be used with an alternative embodiment of masksignal generator 250 such that detector 276 detects the presence of aleakage signal 80 (FIG. 14) and indicates the presence of the leakagesignal 80 to the mask signal generator 250. In the embodimentillustrated in FIG. 15, detector 276 includes a high-inputinstrumentation amplifier (IA) 277 that can unobtrusively monitor thesignals across connection pair 252. Detector 276 could be furtherconfigured to provide an amplifier and/or band filtered replica of anydetected signals. For convenience of illustration, connection 278 isshown to couple detector 276 with interface 264 such that the presenceof a leakage signal 80 detected by detector 276 may be communicated tomask signal generator 250. Connection 278 could alternatively have beencoupled to alternative elements within mask signal generator 250, suchas to processor 262. Any suitable detector 276 which detects leakagesignal 80 may be employed with a mask signal generator 250 withoutdeparting substantially from the spirit and principles of the presentinvention. Furthermore, detector 276 may reside in any convenientlocation as a stand-alone unit, be incorporated with other electricalequipment, or be incorporated as an integral component of a mask signalgenerator 250, without adversely affecting the functionality of the masksignal generator 250 which employs a detector 276. Any such alternativeembodiments of the detector methods and apparatus so employed areintended to be within the scope of this disclosure and be protected bythe accompanying claims for a mask signal generator 250.

5. Alternative Embodiments of a Mask Signal Generator

The mask signal generator 250, as illustrated in FIGS. 13A and 13B, isshown residing as a stand alone component residing in CO22. Such a masksignal generator 250 may be located in other convenient locations. Masksignal generator 250 could also be implemented as a part of MVLtransceiver 60 or in another physical device not shown in FIG. 13. Anysuch variation in location of the mask signal generator 250 could beimplemented without departing substantially from the spirit andprinciples of the mask signal generator 250 of the present invention. Itis intended that all such variations be included herein within the scopeof this disclosure and be protected by the accompanying claims for themask signal generator of the present invention.

As illustrated in FIG. 13B, mask signal generator 250 is transmittingthe mask signal 254 (FIG. 14) into connections 254A–254D. The masksignal 254 could be transmitted into alternative locations and performequally well at masking a leakage signal 80. For example, a mask signalgenerator 250 may generate a mask signal into a plurality of connectionssuch that the plurality of connections could be coupled in convenientlocations to introduce the mask signal 256 (FIG. 14) into the telephonysystem 20″. In this instance, a plurality of output connections could becoupled to a plurality of subscriber loops coupled to a MVL transceiver60 (FIGS. 13A and 13B) or other communication device which iscommunicating with less than the four subscriber loops, or more than thefour subscriber loops. Furthermore, a single mask signal generator 250may transmit a mask signal 256 onto a plurality of subscriber loopswhich may be coupled to more than one MVL transceiver 60 or othercommunication device. Any such variations and modifications in a masksignal generator 250 are intended to be within the scope of thisdisclosure for a mask signal generator 250 and be protected by theaccompanying claims for the mask signal generator 250.

An alternative embodiment of mask signal generator 250 may beconstructed without the inclusion of interface 264, processor 262 andmemory 270. Such a mask signal generator 250 may employ a signalgenerator 260 having a predetermined fixed amplitude and a predeterminedfixed frequency range. Alternatively, a signal generator 260 may have anadjustable amplitude and/or frequency ranges. Such adjustments could beprovided by any commonly employed apparatus, means or method employed inthe art of adjusting signals generated by signal generators. Any suchalternative embodiments of a mask signal generator 250 employing theabove-mentioned variations in a signal generator are intended to bewithin the scope of this disclosure and be protected by the accompanyingclaims for a mask signal generator 250.

6. Embodiments Employing Software with Logic Executed by a Processor

The logic 269 (FIG. 15) of the mask signal generator can be implementedin =software, hardware, or a combination thereof. Portions of the masksignal generator may be implemented in software that may be stored in amemory 270 (FIG. 15) and that is executed by a suitable microprocessor(uP) situated in a personal computer (PC), workstation or otherconvenient location, or by processor 262 residing in mask signalgenerator 250 (FIG. 15). However, instructions defining the softwareportion of the mask signal generator, which each comprise an orderedlisting of executable instructions for implementing logical functions,can be embodied in any computer-readable medium for use by or inconnection with an instruction execution system, apparatus, or device,such as a computer-based system, processor-containing system, or othersystem that can fetch the instructions from the instruction executionsystem, apparatus, or device and execute the instructions.

7. Alternative Embodiments of a Mask Signal

FIGS. 16A–16C illustrate three possible alternative mask signalsgenerated by alternative embodiments of a mask signal generator 250(FIGS. 13A, 13B and 15). Any number of possible variations in a masksignal may be generated by mask signal generator 250. Such a multitudeof possible variations in a mask signal cannot be conveniently describedor illustrated herein. These numerous various alternative embodiments ofa mask signal would each have the functionality of preventing themeaningful detection and amplification of leakage signals. It isintended that all such variations in a mask signal be included withinthe scope of this disclosure and be protected by the accompanying claimsfor the mask signal generator 250.

Signals and/or signal features shown in FIGS. 16A–16C may correspond tosignals and/or signal features shown in FIG. 14. For convenience ofillustration, signals and/or signal features in FIGS. 16A–16C that aresimilar to those in FIG. 14, bear the same reference numerals. Suchsignals and/or signal features having the same reference numerals inFIGS. 16A–16C and FIG. 14 may be considered to be like signals and/orlike signal features, however, since these like numeraled signals and/orsignal features are incidental to the operation of the presentinvention, one skilled in the art should realize that signals and/orsignal features in FIGS. 16A–16C and FIG. 14 need not be identical, asany variations of such signals and/or signal features will not adverselyaffect the functioning and performance of the present invention.Therefore, like signals and/or signal features are like-numbered andwill not be described again in detail.

FIG. 16A illustrates one possible alternative embodiment of a masksignal 280. Mask signal 280 encompasses only a portion of the frequencyrange of leakage signal 80. Also, mask signal 280 encompasses asufficiently broad range of frequency such that leakage signal 80 isrendered substantially undetectable. As shown in FIG. 16A, portions ofleakage signal 80 are not masked. These unmasked portions of leakagesignal 80 are difficult to meaningfully detect and amplify by apotential eavesdropper.

FIG. 16B illustrates another possible alternative embodiment of a masksignal 282. Here, the amplitude of mask signal 282 is not as great asthe amplitude of portions of leakage signal 80. However, a sufficientportion of leakage signal 80 is masked by mask signal 282 such thatleakage signal 80 is difficult to meaningfully detect and amplify by apotential eavesdropper.

FIG. 16C illustrates yet another possible alternative embodiment of amask signal 284. Mask signal 284 includes portions of the mask signal284 a–c which may vary in amplitude and/or frequency. For convenience ofillustration, mask signal 284 is shown having three portions, 284 a, 284b and 284 c. The first portion, 284 a, is seen to have an amplitudeslightly less than the corresponding portion of leakage signal 80. Themiddle portion, 284 b, is seen to have a greater amplitude than thecorresponding portion of leakage signal 80. The third portion, 284 c, isseen to have an amplitude such that the part is less than and anotherpart is greater than the corresponding portion of leakage signal 80. Intotality, the mask signal 284 has been generated such that leakagesignal 80 is difficult to meaningfully detect and amplify by a potentialeavesdropper.

Mask signal 284 may have more than, or less than, the three portions ofmask signal 284 as illustrated in 16C. Furthermore, any portion of masksignal 284 may have amplitudes less than, equal to, or greater than thecorresponding portion of leakage signal 80. Mask signal 284 may beconstructed with any plurality of portions such that the mask signal 284generated is such that leakage signal 80 is difficult to meaningfullydetect and amplify by a potential eavesdropper. Any variations andmodifications in a mask signal 284 are intended to be within the scopeof this disclosure for a mask signal generator 250 and be protected bythe accompanying claims for the mask signal generator 250.

As seen in FIGS. 16A–16C, the mask signal 280, 282 and 284,respectively, have a higher frequency limit which is less than the lowerfrequency F3 of data signal 78. Thus, the mask signals 280, 282 and 284do not significantly interfere with the transmission of data signal 78.Also, the maximum amplitude of mask signals 280, 282 and 284 aresufficiently lower than POTS signal 258 (which corresponds to telephoneconversations on telephone 30D of FIGS. 13A and 13B). Thus, mask signals280, 282 and 284 do not significantly interfere with POTS signal 258.

E. Fourth Embodiment of the Eavesdropping Prevention System and Method:A Multipoint Controllable Line Selection System

1. Controllable Line Selection Unit

FIG. 17 illustrates a controllable line selection unit 300 coupled toone line coupler 302, hereinafter referred to as line coupler A forconvenience. Line coupler A may be configured substantially according tothe previously described line coupler 218A (FIG. 12). Alternatively,line coupler A may be configured according to any coupling device usedto couple a multi-point transceiver (not shown) to a subscriber loop(not shown), thereby providing connectivity back to a customer premises(not shown). Controllable line selection unit 300 has at least acontroller 304, a transmit line selector 306 and a receive line selector308. Transmit line selector 306 has at least two switches 310 and 312.Similarly, the receive line selector 308 has at least two switches 314and 316.

Controllable line selection unit 300 is coupled to a transmitter (notshown) residing in an MVL transceiver (not shown) or similarlyfunctioning communication device. Connection 318 couples thecontrollable line selection unit 300 to the MVL transceiver terminalwhich transmits the positive transmit signal (Tx+). Connection 318 iscoupled to the input of switch 310. Similarly, connection 320 couplesthe controllable line selection unit 300 to the negative transmit signal(Tx−) terminal of the MVL transceiver by connecting to the input ofswitch 312. The controllable line selection unit 300 is coupled to theline coupler A via connections 322 and 324. Connection 322 couples theoutput position A on switch 310 to the Tx+terminal of line coupler A.Connection 324 couples the output position A of switch 312 to the Tx−terminal of line coupler A.

Also, controllable line selection unit 300 is coupled to a receiver (notshown) residing in an MVL transceiver (not shown) or similarlyfunctioning communication device. Connection 326 couples thecontrollable line selection unit 300 to the MVL transceiver terminalwhich receives the positive receive signal (Rx+). Connection 326 iscoupled to the input of switch 314. Similarly, connection 328 couplesthe controllable line selection unit 300 to the negative receive signal(Rx−) terminal of the MVL transceiver by connecting to the input ofswitch 316. The controllable line selection unit 300 is coupled to theline coupler A via connections 330 and 332. Connection 330 couples theoutput position A on switch 314 to the Rx+ terminal of line coupler A.Connection 332 couples the output position A of switch 316 to the Rx−terminal of line coupler A.

Controller 304 provides switch position control signals to switches 310,312, 314 and 316 such that one of a plurality of desired outputpositions is actuated within the switches. For convenience ofillustration in FIGS. 17–19, and for convenience of explaining theoperation and functionality of the control line selection unit 300 andits associated elements, four output positions A, B, C and D are shownresiding in switches 310, 312, 314 and 316. The operation andfunctionality of the present invention is equally applicable to acontrol line selection unit having two, three or more than four outputpositions residing in the switches. Any such alternative embodiments ofa controllable line selection unit 300 are intended to be within thescope of this disclosure and to be protected by the accompanying claimsfor the control line selection unit.

Controller 304 provides the switch position control signal to switch 310via connection 336. Similarly, controller 304 provides a switch positioncontrol signal to switch 312 via connection 338, to switch 314 viaconnection 340 and to switch 316 via connection 342. As will bedescribed hereinafter, controller 304 determines the appropriate switchposition control signal provided to the switches based upon the timingof a time-duplexed communication signal, or another appropriatemultiplexed communication signal, as described below. The required inputsignal for controller 304 is provided over connection 344.

When the MVL transceiver is communicating to a first customer premises(not shown) over a first communication connection, such as, but notlimited to subscriber loop (not shown), coupled to line coupler A, thecontrollable line selection unit 300 provides connectivity between theMVL transceiver and line coupler A. Controller 304 instructs switch 310,residing in transmit line selector 306, to actuate to position A suchthat connection 318 and 322 are coupled together, thereby providingconnectivity for transmission of the Tx+ signal between the MVLtransceiver and the first customer premises, via line coupler A and thefirst communication connection. Concurrently, switch 312 is actuated toposition A so that connection 320 is coupled to connection 324, therebyproviding connectivity for transmission of the Tx− signal between theMVL transmitter and the first customer premises, via line coupler A andthe first communication connection.

Similarly (and concurrently), controller 304 instructs switch 314,residing in receive line selector 308, to actuate to position A suchthat connection 326 and 330 are coupled together, thereby providingconnectivity for transmission of the Rx+ signal between the firstcustomer premises and the MVL transceiver, via line coupler A and thefirst communication connection. Concurrently, switch 316 is actuated toposition A so that connection 328 is coupled to connection 332, therebyproviding connectivity for transmission of the Rx− signal between thefirst customer premises and the MVL transceiver, via line coupler A andthe first communication connection. Thus, when all four switches (310,312, 314 and 316) are actuated to position A, the MVL transceiver(transmitter and receiver) is coupled to the first customer premises.

2. Time Duplexed Communication Signal

FIG. 18 illustrates an exemplary four channel time-duplexedcommunication signal 350. As is well known in the art, a plurality ofcommunication signals may be time-division multiplexed for transmissionover a single communication connection by allocating each communicationsignal to a predefined communication channel. The communication channelis assigned a portion of a repeatable time period. In the illustrativeexample of the time-duplexed communication signal 350 illustrated inFIG. 18, four individual communication signals are being transmittedover a single communication connection (not shown). The firstcommunication signal is assigned to channel 1. Similarly, the secondcommunication signal is assigned to channel 2, the third communicationsignal is assigned to channel 3 and the fourth communication signal isassigned to channel 4. For convenience of illustration, channels 1, 2, 3and 4 are shown to have a time period allocation approximately equal toone-quarter of the repeatable time period 352, and are ordered in thesequence as shown in FIG. 18. Thus, the first communication signal iscommunicated during the time period allocated to channel 1. Likewise,the second, third and fourth communication signals are communicatedduring the times allotted to channels 2, 3 and 4, respectively.

During the channel 1 time period, the controller 300 (FIG. 17) actuatesswitches 310, 312, 314 and 316 to position A (see FIG. 17). When thechannel 1 time period ends and the channel 2 time period begins,controller 304 provides a switch position control signal to switches310, 312, 314 and 316 such that the switches are actuated to position B(see FIG. 17). As described hereinafter, position B providesconnectivity between the MVL transceiver and a second line coupler,thereby providing connectivity to a second communication connectionconnecting to a second customer premises. Thus, a second communicationsignal is being communicated by the MVL transceiver through thecontrollable line selection unit 300 (which has actuated switches 310,312, 314 and 316 to position B), thereby providing connectivity to thesecond customer premises.

Similarly, during the channel 3 time period, controller 304 actuatesswitches 310, 312, 314 and 316 to position C. Thus, the thirdcommunication signal is being communicated between the MVL transceiverand a third customer premises through the controllable line selectionunit 300 (which has actuated switches 310, 312, 314 and 316 to positionC to provide connectivity via a third line coupler and a thirdcommunication connection).

Likewise, during the channel 4 time period, controller 304 actuatesswitches 310, 312, 314 and 316 to position D. Thus, the fourthcommunication signal is being communicated between the MVL transceiverand a fourth customer premises through the controllable line selectionunit 300 (which has actuated switches 310, 312, 314 and 316 to positionD to provide connectivity via a fourth line coupler and a fourthcommunication connection).

When the channel 4 time period ends, a new repeatable time period 352begins with channel 1. Controller 304 then provides a switch positioncontrol signal to switches 310, 312, 314 and 316 such that the switchesare actuated to position A. The sequence of providing the switchposition control signals by controller 304 to switches 310, 312, 314 and316 is repeated for channels 2, 3 and 4, thereby providing connectivityto four different customer premises at the appropriate times. That is,during the repeating channel 1 time period, the first communicationsignal is communicated between the MVL transceiver and the firstcustomer premises (via the first subscriber loop) through line coupler A(switches 310, 312, 314 and 316 are actuated to position A as shown inFIG. 17). Then, controller 304 actuates switches 310, 312, 314 and 316to position B such that the second communication signal is communicatedbetween the MVL transceiver and a second customer premises during thechannel 2 time period. Next, controller 304 actuates switches 310, 312,314 and 316 to position C such that the third communication signal iscommunicated between the MVL transceiver and a third customer premisesduring the channel 3 time period. Finally, controller 304 actuatesswitches 310, 312, 314 and 316 to position D such that the fourthcommunication signal is communicated between the MVL transceiver and afourth customer premises during the channel 4 time period.

The operation of controller 304 as described above in association withthe time-duplexed communication signal 350 (FIG. 18) requires that aninput signal be provided to controller 304 over connection 344 (FIG.17). The input signal to controller 304 must have means to identify thetransition between the allocated time periods for each channel in thetime-duplexed communication signal 350. For example, and as illustratedin FIG. 18, when the repeatable time period 354 ends at the conclusionof the channel 4 time period, the input signal provided to controller304 should indicate the end of the channel 4 time period of repeatabletime period 354 (or alternatively, the beginning of the channel 1 timeperiod of repeatable time period 352). Controller 304 can then provide aswitch position control signal to switches 310, 312, 314 and 316 suchthat the switches are actuated to position A. Similarly, at the end ofthe channel 1 time period of repeatable time period 352 (oralternatively, the beginning of the channel 2 time period of repeatabletime period 352), controller 304 should provide a switch positioncontrol signal to switches 310, 312, 314 and 316 such that the switchesare actuated to position B. Then, at the end of the channel 2 timeperiod of repeatable time period 352 (or alternatively, the beginning ofthe channel 3 time period of repeatable time period 352), controller 304should provide a switch position control signal to switches 310, 312,314 and 316 such that the switches are actuated to position C. Next, atthe end of the channel 3 time period of repeatable time period 352 (oralternatively, the beginning of the channel 4 time period of repeatabletime period 352), controller 304 should provide a switch positioncontrol signal to switches 310, 312, 314 and 316 such that the switchesare actuated to position D. The process described above repeats at theend of the channel 4 time period of repeatable time period 352 (oralternatively, the beginning of the channel 1 time period of repeatabletime period 356), as controller 304 provides a switch position controlsignal to switches 310, 312, 314 and 316 such that the switches areactuated to position A.

As described above, controllable line selection unit 300 was illustratedas having switches with four output positions. A controllable lineselection unit 300 may employ switches having two, three or more thanfour output positions. Such a control line selection unit 300 would beemployed in a communication system communicating a time-duplexedcommunication signal having two, three or more than four communicationchannels, respectively. However, a controllable line selection unit 300employing switches having more output positions than the availablenumber of communication channels could be implemented in a communicationsystem. For example, a controllable line selection unit 300 may employswitches having five output positions; A, B, C, D and E. In thisexample, channels one, two, three and four could be assigned to outputpositions A, B, C and D, respectively, as described above. Position Ewould not be assigned to a communication channel and remain inactive.That is, in the repeating sequence wherein the switch positions wereactuated according to the assigned time channels, output position Ewould be simply bypassed. This technique may be particularlyadvantageous in communication systems in which the nature of thetime-duplexed communication signal might change in the future, or whenit may be desirable to have flexibility in reassigning channels todifferent output positions. Alternatively, channels one, two, three andfour could be assigned to output positions A, B, C and E (therebybypassing position D). Or, a fifth channel (assigned to output positionE) may be added to the communication signal.

3. Controllable Line Selection Unit Coupled to Four POTS Connections

FIG. 19 illustrates selected components of a preferred embodiment of theexemplary controllable line selection unit 300 (FIG. 17) having fourswitch positions coupled to four line couplers (360, 362, 364 and 366).For convenience of illustration, the selected components of thecontrollable line selection unit 300, are illustrated without regard toactual orientation or placement in an actual operating communicationsystem. Rather, the orientation of the selected components shown in FIG.19 is based upon the need to conveniently describe the operation andfunctionality of a controllable line selection unit 300 and toillustrate the connections between an exemplary transmitter 368, anexemplary receiver 370 residing in a MVL transceiver (not shown) andfour exemplary line couplers 360, 362, 364 and 366. The first linecoupler 360 is hereinafter referred to as line coupler A, forconvenience, because line coupler A corresponds to position A ofswitches 310 and 312 (and switches residing in receive line selector308, which are not shown in FIG. 19). Similarly, the second line coupler362 is hereinafter referred to as line coupler B (because line coupler Bcorresponds to switch position B), the third line coupler 364 ishereinafter referred to as line coupler C (because line coupler Ccorresponds to switch position C), and the fourth line coupler 366 ishereinafter referred to as line coupler D (because line coupler Dcorresponds to switch position D).

Line coupler A is coupled to the tip and ring conductors of a firstcommunication connection, such as, but not limited to, a subscriber loop(not shown). Referring back to the general communication systemtopography illustrated in FIG. 3, subscriber loop 26A corresponds to the“first” communication connection described above. Subscriber loop 26Aprovides connectivity between the MVL transceiver 60 and customerpremises 24A (which corresponds to the “first” customer premisesdescribed above). In the communication system illustrated generally inFIG. 3, line coupler A would be coupled at a convenient location onconnection 62A.

Line coupler B is similarly connected to the tip conductor and the ringconductor of a second communication connection (not shown). Referringback to the general communication system topography illustrated in FIG.3, subscriber loop 26B corresponds to the “second” communicationconnection described above. Subscriber loop 26B provides connectivitybetween the MVL transceiver 60 and customer premises 24B (whichcorresponds to the “second” customer premises described above). In thecommunication system illustrated generally in FIG. 3, line coupler Bwould be coupled at a convenient location on connection 62B.

Likewise, line coupler C provides coupling to a third communicationconnection (not shown) and line coupler D provides coupling to a fourthcommunication connection (not shown). Referring back to the generalcommunication system topography illustrated in FIG. 3, subscriber loop26C corresponds to the “third” communication connection described aboveand subscriber loop 26D corresponds to the “fourth” communicationconnection. Subscriber loop 26C provides connectivity between the MVLtransceiver 60 and customer premises 24C (which corresponds to the“third” customer premises described above). Subscriber loop 26D providesconnectivity between the MVL transceiver 60 and customer premises 24D(which corresponds to the “fourth” customer premises described above).In the communication system illustrated generally in FIG. 3, linecoupler C would be coupled at a convenient location on connection 62C,and line coupler D would be coupled at a convenient location onconnection 62D.

Returning now to FIG. 19, the exemplary MVL transmitter 368 is shown ashaving a signal generator 372 and two amplifiers 376 and 378. Also shownin MVL transmitter 368 are a plurality of resistors R. The components ofthe MVL transmitter 368 as shown in FIG. 19 are intended to demonstrateone possible embodiment of a transmitter and the associated connectionsto line couplers A–D. MVL transmitter 368 generates a full duplexcommunication signal such that the amplifier 376 outputs the Tx+ signaland amplifier 378 outputs the Tx− signal.

The Tx+ signal from MVL transmitter 368 is provided to switch 310 viaconnection 380. The Tx− signal is provided to switch 312 via connection382. When the switches are actuated to the A for communication of theTx+ signal position (corresponding to channel 1 in the example above)connectivity to line coupler A is provided from the A position in switch310 via connections 384 and 386. Likewise, when switch 312 is actuatedto the A position, connectivity to line coupler A for communication ofthe TX− signal is provided over connections 388 and 390. Switches (notshown) in the receive line selector 308 are also actuated to position Asuch that any received signals (Rx+ and Rx−) may be detected overconnections 392 and 394. As described above, controller 304 has providedswitch position control signal to switch 310 and switch 312, andswitches residing in receive line selector 308 to actuate to position Avia connection 396.

For convenience of illustration, controller 304 is coupled to theswitches via the single connection 396. Such a controller employing asignal connection to couple to all switches would provide the sameswitch position control signal to each switch. However, a controller 304according to FIG. 17 which employs separate connections to each switch,could have been equally employed here without departing substantiallyfrom the operation and functionality of the present invention.

As the communication signal 350 (FIG. 18) is cycling from channel 1 tochannel 2 to channel 3 to channel 4, controller 304 provides switchposition control signals to the switches to actuate the switches topositions A, B, C and D, respectively. When communications are occurringduring the channel 4 time period, switches are actuated to position D asdescribed above. As illustrated in FIG. 19, line coupler D is nowcoupled to the MVL transmitter 368 via connections 398, 400, 402 and 404as shown. Similarly, receive line selector 308 would be coupled to linecoupler D via connections 406 and 408. Receive line selector 308 iscoupled to the MVL receiver 370 such that the Rx+ signal is providedover connection 410 to the MVL receiver 370 and the Rx− signal isprovided over connection 412 to MVL receiver 370.

The embodiment illustrated in FIG. 19 is intended to be an example ofone of many possible communication systems which could effectivelyemploy a controllable line selection unit. The present invention isequally applicable to communication systems which employ differentconfigurations of transmitters, receivers and/or line couplers. Any suchcommunication system employing a controllable line selection unit isintended to be within the scope of this disclosure and to be protectedby the accompanying claims for the controllable line selection unit.

4. Operation of the Preferred Embodiment with a Time DuplexedCommunication Signal to Prevent Propagation of Leakage Signals

One skilled in the art, upon consideration of the elements and operatingprocesses for a controllable line selection unit 300, as described abovein relation to FIGS. 17–19, will realize that when the firstcommunication signal is being communicated during the channel 1 timeperiod, the MVL transceiver would be coupled to the first communicationconnection (through line coupler A) because the controller 304 hasprovided the necessary switch position control signal to actuateswitches 310, 312, 314 and 316 to position A. During this time, the MVLtransmitter 368 and the MVL receiver 370, are isolated from the othercommunication connections. Thus, the physical isolation from the othercommunication connections prevents a leakage signal generated bycommunication signals on the second, third and/or fourth communicationconnections from propagating onto the first communication connection.

When the second communication signal is being communicated during thechannel 2 time period (FIG. 18), the MVL transceiver would be coupled tothe second communication connection (through line coupler B) because thecontroller 304 has provided the necessary switch position control signalto actuate switches 310, 312, 314 and 316 to position B. During thistime, the MVL transmitter 368 and the MVL receiver 370, are isolatedfrom the other communication connections. Thus, the physical isolationfrom the other communication connections prevents a leakage signalgenerated by communication signals on the first, third and/or fourthcommunication connections from propagating onto the second communicationconnection.

Similarly, when the third communication signal is being communicatedduring the channel 3 time period (FIG. 18), the MVL transceiver would becoupled to the third subscriber loop (through line coupler C) becausethe controller 304 has provided the necessary switch position controlsignal to actuate switches 310, 312, 314 and 316 to position C. Duringthis time, the MVL transmitter 368 and the MVL receiver 370, areisolated from the other communication connections. Thus, the physicalisolation from the other communication connections prevents a leakagesignal generated by communication signals on the first, second and/orfourth communication connections from propagating onto the thirdcommunication connection.

Finally, when the fourth communication signal is being communicatedduring the channel 4 time period (FIG. 18), the MVL transceiver would becoupled to the fourth communication connection (through line coupler D)because the controller 304 has provided the necessary switch positioncontrol signal to actuate switches 310, 312, 314 and 316 to position D.During this time, the MVL transmitter 368 and the MVL receiver 370, areisolated from the other communication connections. Thus, the physicalisolation from the other communication connections prevents a leakagesignal generated by communication signals on the first, second and/orthird communication connections from propagating onto the fourthcommunication connection.

Controllable line selection unit 300, as described in FIGS. 17–19,employ switches having four output positions A–D. As noted above, thecontrollable line selection unit 300 may employ switches having two,three or more than four output positions. Such an embodiment of acontrollable line selection unit 300 may be particularly desirable whenthe communication system has two, three or more than four communicationconnections connecting back to customer premises to which thecontrollable line selection unit 300 is to provide coupling to. Forexample, in a controllable line selection unit 300 employing switcheshaving five output positions A–E, the controllable line selection unit300 could be connected to five different line couplers, therebyproviding for connectivity to five different customer premises. Byappropriately assigning communication channels to the desired switchoutput positions A–E, connectivity to the five customer premises couldbe provided as required. For example, in the situation of acommunication signal having only four channels, channel 1 could beassigned to output position A, channel 2 assigned to output position B,channel 3 assigned to output position C, and channel 4 assigned tooutput position D. Alternatively, channel 4 might be assigned to outputposition E (rather than output position D). Such a configuration may beparticularly advantageous when customers are changing service levelswith their service providers or in situations where the network topologyis being altered. Furthermore, it is not necessary that the channelassignments to be made in the sequential order of the switch outputpositions. That is, channels 1, 2, 3 and 4 might be assigned to channelsA, C, E and B, respectively. Or, the channels may be assigned to anydesired output switch position. Furthermore, a single output switchposition may be assigned multiple channels. For example, channels oneand three might be assigned to switch output position C. Any suchalternative embodiments of a controllable line selection unit 300 asdescribed above, are intended to be within the scope of this disclosureand to be protected by the accompanying claims for a controllable lineselection unit.

5. Controller System Components

FIG. 20 illustrates selected components which may be employed in acontroller 304 implemented as part of a controllable line selection unit300 (FIG. 17). Controller 304 has at least a processor 420 incommunication with a memory 422 via connection 424. Logic 426 resides inmemory 422. Processor 420 is shown to have at least four control signaloutput connections 336, 338, 340 and 342 (see also FIG. 17). As notedabove, the required input signal for controller 304 is provided overconnection 344, shown coupled to processor 420 in FIG. 20. Processor 420is detecting the channel transitions previously described for FIG. 18.Processor 420 is also coupled to an external device 428 via connection430.

External device 428 provides information regarding the channelassignments to switch output positions to processor 420. Processor 420stores the switch position and channel assignment information in memory422. External device 428 may be any type of suitable device whichprovides the necessary information to processor 420. For example,external device 428 may be a keyboard used by an operator to manuallyprovide the switch position and channel assignment information toprocessor 420. Alternatively, external device 428 may be anotherprocessing system which provides the necessary information to processor420. One skilled in the art will appreciate that the external device 428may be implemented using well-known techniques commonly employed in theart. Memory 422 may be a composite memory having a variety of differenttypes of memory elements, such as, read only memory (ROM) and/or randomaccess memory (RAM) or other suitable memory elements. Thus, a detailedexplanation of the elements, components, functionality and/or operationof the external device 428 and memory 422 is not provided herein as sucha detailed explanation is not necessary to the understanding of theoperation and functionality of a controllable line selection unit 300.It is intended that all such variations in the type of external device428 and memory 422 employed be within the scope of this disclosure andto be protected by the accompanying claims for a controllable lineselection unit.

For convenience of illustration in FIG. 20, processor 420, logic 426 andmemory 422 are shown residing in controller 304. These components mayreside in alternative convenient locations outside of the controller304, as components of other systems, or as stand alone dedicatedelements without adversely affecting the operation and functionality ofthe controllable line selection unit. Furthermore, processor 420 isshown for convenience of illustration as directly providing the switchposition control signals to the switches via connections 336, 338, 340and 342. In alternative embodiments, intermediate devices (not shown)may be employed such that the switch position control signal generatedby processor 420 is configured to a suitable signal for the actuation ofthe switches residing in a controllable line selection unit 300 (FIG.17). Any such alternative embodiments of a controllable line selectionunit 300 are intended to be within the scope of this disclosure and tobe protected by the accompanying claims for a controllable lineselection unit.

6. Controllable Line Selection Unit Operation Flow Chart

FIG. 21 is a flow chart 440 illustrating the operation of the logic 426of FIG. 21 as applied to a method for controlling switch outputpositions in a controllable line selection unit 300 (FIG. 17). The flowchart of FIG. 21 shows the architecture, functionality, and operation ofa possible implementation of the software for implementing the logic426. In this regard, each block may represent a module, segment orportion of code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat in some implementations, the functions noted in the blocks mayoccur out of the order noted in FIG. 21 or may include additionalfunctions without departing significantly from the functionality of thecontrollable line selection unit 300. For example, two blocks shown insuccession in FIG. 21 may in fact be executed substantiallyconcurrently, the blocks may sometimes be executed in reverse order, orsome of the blocks may not be executed in all instances, depending uponthe functionality involved, as will be further clarified below. All suchmodifications and variations are intended to be included within thescope of this disclosure and to be protected by the accompanying claimsfor a controllable line selection unit. In block 442, logic 426determines the current channel of the communication signal. For example,communications assigned to channel 1 (see FIG. 18) may be currently inprogress. Once the current channel is determined, the associated switchposition assignment is read from memory 422 (FIG. 20). In thisillustrative example, the next channel would be channel 2 and the switchposition assignment for channel 2 is switch position B.

The communication signal is monitored to detect the channel transitionat block 446. In this illustrative example, the logic 426 would bedetecting the transition from channel 1 to channel 2 (see also FIG. 18).Then, controller 304 (FIG. 17) would generate a switch position controlsignal to actuate the switches to the next assigned position, here,position B (see also FIG. 18). Then, logic 426 would determine whetheror not the communication signal is on-going (the YES condition) or ifthe communication signal has ended (the NO condition). If thecommunication signal has not ended (the YES condition) the processreturns to block 444 and continues accordingly. If the communicationsignal has ended (the NO condition) the process ends.

7. Alternative Embodiments of a Controllable Line Selection Unit

As noted above, alternative embodiments of the controllable lineselection unit may be employed as a means for isolating communicationconnections such that leakage signals generated from communicationsignals do not propagate onto other communication connections. Theoperation and functionality of the controllable line selection unit wasdescribed above with respect to a communication signal having fourportions assigned to four different channels. For convenience ofillustration, and for convenience of explaining the functionality andoperation of a controllable line selection unit, four channels wereselected with each channel having approximately an equal time period. Asnoted above, a controllable line selection unit will perform equallywell on a communication signal having two channels, three channels ormore than four channels. Furthermore, the numbers of channels mayperiodically change and/or the time periods of each channel may not beapproximately equal. Alternative embodiments of a controllable lineselection unit may be easily configured to detect any plurality ofchannels, and since the controllable line selection unit detectstransitions from one channel to the next channel, the time periodsassociated with each channel need not be equal. Furthermore, thecontrollable line selection unit may be configured to assign the sameswitch position to one or more of the plurality of channels. A detailedexplanation of the components, operation and functionality of suchalternative embodiments is not described herein as one skilled in theart can readily appreciate operation and functionality of suchalternative embodiments, and therefore easily practice any suchalternative embodiments of a controllable line selection unit.Furthermore, such alternative embodiments are too numerous toindividually describe in a single application specification. Any suchalternative embodiments of a controllable line selection unit areintended to be within the scope of this disclosure and to be protectedby the accompanying claims for the controllable line selection unit.

For convenience of illustration and for convenience of explaining theoperation and functionality of the controllable line selection unit, anexemplary four channel time-division multiplexed communication signalwas described. The controllable line selection unit will perform equallywell with other types of communication signals, such as, but not limitedto, a time division multiplexed echo canceled communication signal, atime-division multiplexed frequency-division communication signal, atime-division time-compressed communication signal or other suitablecommunication signal having at least two time-multiplexed channels. Thecontrollable line selection unit detects transitions in a communicationsignal and activates switches to predefined positions based upon thedetected transitions. The detected transitions correspond to portions ofa communication signal that are intended to be communicated to one of aplurality of different locations and/or different devices. Any suchalternative embodiments of a controllable line selection unit configuredto detect transitions in a communication signal and is configuredactuate switch positions accordingly, are intended to be within thescope of this disclosure and to be protected by the accompanying claimsfor the controllable line selection unit.

In some applications, it may be desirable to detect periods ofcommunication inactivity (no transmit signal or receive signal) in achannel. One alternative embodiment of a controllable line selectionunit detects such periods of inactivity in a channel and selectivelyreassigns the time allocated to the unused channel to another channelwhich is actively communicating. For example, referring to FIG. 18,during certain times channel 3 may be inactive (no communicationspresent during the time allocated to channel 3). Logic residing in thecontroller may selectively reallocate the time allocated to channel 3 toanother active channel. For example, communications during the timeallocated to channel 4 may be very active. Logic may then expand thetime period allocated to channel 4 (by reducing the time periodallocated to channel 3), thereby increasing the amount of data flowoccurring over channel 4. Another embodiment operates such that theswitch position assigned to channel 4 is concurrently assigned tochannel 3.

FIG. 22 illustrates an alternative embodiment of a controllable lineselection unit 460. Controllable line selection unit 460 is coupled to aseparate transmitter 462 and a separate receiver 464. Similar to thecontrollable line selection unit 300 (FIG. 17), the controllable lineselection unit 460 has a controller 466, a transmit line selector 468and a receive line selector 470. The transmit line selector 468 has afirst switch 472 and a second switch 474. Similarly, receive lineselector 470 has a first switch 476 and a second switch 478.

Switches 472, 474, 476 and 478 have four output switch positions M, N, 0and P. Similar to the controllable line selection unit 300 (FIG. 17),the switches 472, 474, 476 and 478 residing in controllable lineselection unit 460 may be coupled to up to four line couplers. Forconvenience of explaining the operation and functionality of thecontrollable line selection unit 460, a first coupler 480 and a secondcoupler 482 are shown. Line coupler 480 is hereinafter referred to asline coupler M (because line coupler M corresponds to switch position M)and line coupler 482 is hereinafter referred to as line coupler P(because line coupler P corresponds to switch position P).

Controller 466 detects the above-described transitions between channelsin a communication signal, via connection 484, and provides theappropriate switch position control signals to switch 472, viaconnection 486, to switch 474 via connection 488, to switch 476 viaconnection 490 and to switch 478 via connection 492.

The controllable line selection unit 460, configured according to FIG.22, provides for the simultaneous transmission of a first communicationsignal (Tx+ and Tx−) to a selected one of the plurality of linecouplers, and the receiving of a second communication signal (Rx+ andRx−) by receiver 464. As illustrated in FIG. 22, transmitter 462 iscoupled to line coupler M. Receiver 464 is coupled to a different one ofthe plurality of line couplers, having line coupler P.

Connection 494 couples switch 472 with the transmitter 462 so that theTx+ signal may be transmitted over a first communication connectioncoupled to line coupler M. Similarly, connection 496 couples switch 474to transmitter 462 for transmission of the Tx− signal. Connection 498couples switch 476 to the receiver 464 so that the Rx+communicationsignal can be received over a second communication connection coupled toline coupler P. Likewise, connection 500 couples switch 478 to thereceiver 464 so that the Rx− communication signal can be received.

The operation and functionality of the controllable line selection unit460 is described by way of a simplified illustrative example, and isillustrated accordingly in FIG. 22. Controllable line selection unit 460provides for the simultaneous communication of two communication signalsas follows. A first communication signal being communicated during afirst channel (time period) is transmitted to line coupler M viaconnections 502 and 504. That is, controller 466 has actuated switches472 and 474 to the M position during this first channel. Simultaneously,controller 466 has actuated switches 476 and 478 to the P position suchthat line coupler P is coupled to receiver 464. Receiver 464 isreceiving a second communication signal from line coupler P, viaconnections 506 and 508, during this first channel.

When transmitter 462 is to transmit to a different location, a channeltransition is detected by controller 466 and switches 472 and 474 areactuated to a different switch position. Likewise, controller 466 willactuate switches 476 and 478 to a different position when a channeltransition in the second communication signal is detected. Generally,the switch positions in switch 472 and switch 474 (which aretransmitting a first communication signal to a pre-selected linecoupler) would not be the same as the switch positions in switch 476 andswitch 478 (which are configured to couple receiver 464 to a second oneof the plurality of line couplers).

The operation and functionality of the controllable line selection unitmay be implemented using any commonly available type of communicationconnection switcher. The line switching functions performed by suchswitch(es), controlled by a processor or other actuating device, may beimplemented with any type of electronic, solid state or firmware typeswitching device or means commonly employed in the art. Such processorbased switch(es) in an (alternative) embodiment of the controllable lineselection unit would be implemented by a combination of software andfirmware using components and methods commonly employed in the art ofswitching electrical devices. It is intended that all suchimplementations of switch(es), and their associated control means, beincluded herein within the scope of this disclosure and be protected bythe accompanying claims for the controllable line selection unit.

One such alternative embodiment of a controllable line selection unit510 is illustrated in FIG. 23. The controllable line selection unit 510is coupled to a controller 466, transmitter 462 and receiver 464 in asimilar manner as shown with the controllable line selection unit 460shown in FIG. 22. However, when the controller actuates switches 512 and514, broadcast transmission switches, all line couplers connected toswitch positions M, N, O and P are simultaneously coupled to thetransmitter 462, thereby providing broadcast message capability. Thus, asingle message will be transmitted to all connected line couplers. Suchan embodiment could be overlaid on top of any other previously describedembodiment of a controllable line selection unit. That is, the M, N, Oand P switch positions of switch 512 would be coupled to the M, N, O andP switch positions of switch 472 (FIG. 22), respectively. Or, thecontrollable line selection unit 510 could be implemented as a standalone system, which would be particularly suitable for a communicationsystem not having POTS signals or having dedicated communicationconnections, such as, but not limited to, private data subscriber loops.

Switches 516 and 518 could be similarly configured to switches 512 and514. Switches 516 and 518, broadcast receiver switches, wouldsimultaneously couple the line couplers to receiver 464 upon activationof switches 516 and 518 by controller 466, thereby providing for thereception of broadcast transmissions.

8. Additional Benefits Realized from a Controllable Line Selection Unit

Controllable line selection unit 460, as described above and illustratedin FIG. 22, provides for the simultaneous transmission of a firstcommunication signal and the receiving of a second communication signal.When operating in this manner, two communication signals may besimultaneously communicated, thereby increasing the overall efficiencyof the communication system in which the controllable line selectionunit 460 has been implemented. Furthermore, the receiver 464 will beable to receive a communication signal that is free from possibleinterference created by the components residing in a transmitter 462because the transmitter 462 is completely isolated from the componentsresiding in receiver 464. Likewise, transmitter 462 may be configured totransmit a communication signal without considering the requirements ofthe receiver 464, which generally detects a much weaker receivedcommunication signal.

F. Alternative Embodiments Implemented on Other Communication Systems

Furthermore, the preferred embodiments of a connection sharingmultipoint POTS splitter with the LS blocking splitter, amplifier-basedcoupler, mask signal generator and controllable line selection unit areillustrated and described in the context of a DSL communicationsnetwork. However, the concepts and principles of the LS blockingsplitter, amplifier-based coupler, mask signal generator andcontrollable line selection unit are equally applicable to othercommunication formats, such as, but not limited to ADSL, RADSL, MVL,VDSL or a combination of systems having segments employing differentformats for each segment.

It should be emphasized that the above-described “embodiments” of the LSblocking splitter, amplifier-based coupler, mask signal generator andcontrollable line selection unit, particularly, any “preferred”embodiments, are merely possible examples of implementations, merely setforth for a clear understanding of the principles of the LS blockingsplitter, amplifier-based coupler, mask signal generator andcontrollable line selection unit. Many variations and modifications maybe made to the above-described embodiment(s) of the LS blockingsplitter, amplifier-based coupler, mask signal generator andcontrollable line selection unit without departing substantially fromthe spirit and principles of the LS blocking splitter, amplifier-basedcoupler, mask signal generator and controllable line selection unit. Forexample, the principles of the LS blocking splitter, amplifier-basedcoupler, mask signal generator detailed and controllable line selectionunit herein are similarly applicable to other communication servicessuch as, for example but not limited to, ADSL. All such modificationsand variations are intended to be included herein within the scope ofthe LS blocking splitter, amplifier-based coupler, mask signal generatorand controllable line selection unit, and be protected by the claimsthat follow.

1. A system for attenuating leakage signals in a communication system, comprising: a multiple virtual line (MVL) transmitter configured to provide signals to a plurality of subscriber loops, each subscriber loop having a tip connection and a ring connection, the MVL transmitter having a first transmit output (Tx+) and a second transmit output (Tx−): an amplifier-based coupler coupled between the MVL transmitter and the plurality of subscriber loops, the amplifier-based coupler comprising: a first amplifier having an output at least coupled to each tip connection of in the plurality of subscriber loops and at least one input coupled to the Tx+ output of the MVL transmitter, and configured to have a nearly-zero impedance characteristic; and a second amplifier having an output at least coupled to each ring connection in the plurality of subscriber loops and at least one input coupled to the Tx− output of the MVL transmitter, and configured to have a nearly-zero impedance characteristic, such that at least one leakage signal originating on a first subscriber loop in said plurality of subscriber loops cannot propagate from subscriber loop to a second subscriber loop in said plurality of subscriber loops.
 2. The system of claim 1, wherein at least one of said plurality of amplifiers is configured as a negative feedback amplifier.
 3. The system of claim 1, further comprising a second plurality of amplifiers, said second plurality of amplifiers coupled between a second plurality of subscriber loops and said MVL transmitter.
 4. The system of claim 1, wherein at least one of said plurality of subscriber loops is a digital subscriber loop.
 5. The system of claim 1, wherein said first subscriber loop is physically coupled to said second subscriber loop.
 6. The system of claim 1, wherein said plurality of subscriber loops are physically coupled together.
 7. The system of claim 1, wherein said plurality of subscriber loops are physically coupled to said MVL transmitter.
 8. The system of claim 1, wherein said MVL transmitter time multiplexes the signals onto a single channel.
 9. The system of claim 1, wherein said MVL transmitter frequency multiplexes the signals onto a plurality of channels.
 10. The system of claim 1, wherein said MVL transmitter is a signal multiplexing communication device.
 11. The system of claim 1, wherein said MVL transmitter is further configured to concurrently provide signals to said plurality of subscriber loops.
 12. A method for shunting leakage signals in a communication system, the method comprising the steps of: receiving a differential digital signal, said differential digital signal composed of a plurality of subscriber signals, each subscriber signal associated with a different subscriber; coupling a first amplifier between the positive differential digital signal and each tip connection in a plurality of subscriber loops, said first amplifier having a nearly-zero impedance characteristic; coupling a second amplifier between the negative differential digital signal and each ring connection in the plurality of subscriber loops, said second amplifier having a nearly-zero impedance characteristic; and shunting at least one leakage signal originating on a first subscriber loop in the plurality of subscriber loops away from a second subscriber loop in the plurality of subscriber loops.
 13. The method of claim 12, wherein said differential digital signal time multiplexes said plurality of subscriber signals onto a single channel.
 14. The method of claim 12, wherein said differential digital signal frequency multiplexes said plurality of subscriber signals onto a plurality of channels.
 15. The method of claim 12, wherein said differential digital signal concurrently provides voice service to each of the subscribers.
 16. The method of claim 12, wherein said differential digital signal concurrently provides voice service to each of the subscribers.
 17. A system for shunting leakage signals in a communication system, comprising: means for receiving a differential digital signal from a digital device, said differential digital signal composed of a plurality of subscriber signals, each subscriber signal associated with a different subscriber, said differential digital signal concurrently providing voice service to each of the subscribers; means for coupling a first amplifier between the positive differential digital signal and each tip connection in a plurality of subscriber loops, said first amplifier having a nearly-zero impedance characteristic; means for coupling a second amplifier between the negative differential digital signal and each ring connection in the plurality of subscriber loops, said second amplifier having a nearly-zero impedance characteristic; and means for shunting such that at least one leakage signal originating on a first subscriber loop in the plurality of subscriber loops away from a second subscriber loop in the plurality of subscriber loops coupled to said digital device.
 18. The system of claim 17, wherein said means for coupling the first amplifier further couples said second subscriber loop to said shunting means.
 19. The system of claim 17, wherein said differential digital signal time multiplexes said plurality of subscriber signals onto a single channel.
 20. The system of claim 17, wherein said differential digital signal frequency multiplexes said plurality of subscriber signals onto a plurality of channels. 